Power-Programmierung/CD1.mdf/perl/perl40/perlman.txt

Text File | 1993-03-14 | 207KB | 5,210 lines

 PERL(1)                                 PERL(1)


 NAME
      perl - Practical Extraction and Report Language

 SYNOPSIS
      perl [options] filename args

 DESCRIPTION
      Perl is an interpreted language optimized for scanning arbitrary text
      files, extracting information from those text files, and printing
      reports based on that information.  It's also a good language for many
      system management tasks.    The language is intended to be practical
      (easy to use, efficient, complete) rather than beautiful (tiny,
      elegant, minimal).  It combines (in the author's opinion, anyway) some
      of the best features of C, sed, awk, and sh, so people familiar with
      those languages should have little difficulty with it.  (Language
      historians will also note some vestiges of csh, Pascal, and even
      BASIC-PLUS.) Expression syntax corresponds quite closely to C
      expression syntax.  Unlike most Unix utilities, perl does not
      arbitrarily limit the size of your data--if you've got the memory,
      perl can slurp in your whole file as a single string.  Recursion is of
      unlimited depth.    And the hash tables used by associative arrays grow
      as necessary to prevent degraded performance.  Perl uses sophisticated
      pattern matching techniques to scan large amounts of data very
      quickly.    Although optimized for scanning text, perl can also deal
      with binary data, and can make dbm files look like associative arrays
      (where dbm is available).  Setuid perl scripts are safer than C
      programs through a dataflow tracing mechanism which prevents many
      stupid security holes.  If you have a problem that would ordinarily
      use sed or awk or sh, but it exceeds their capabilities or must run a
      little faster, and you don't want to write the silly thing in C, then
      perl may be for you.  There are also translators to turn your sed and
      awk scripts into perl scripts.  OK, enough hype.

      Upon startup, perl looks for your script in one of the following
      places:

      1.  Specified line by line via -e switches on the command line.

      2.  Contained in the file specified by the first filename on the
      command line.  (Note that systems supporting the #! notation
      invoke interpreters this way.)

      3.  Passed in implicitly via standard input.  This only works if there
      are no filename arguments--to pass arguments to a stdin script you
      must explicitly specify a - for the script name.

      After locating your script, perl compiles it to an internal form.  If
      the script is syntactically correct, it is executed.


      Options

      Note: on first reading this section may not make much sense to you.
      It's here at the front for easy reference.

      A single-character option may be combined with the following option,
      if any.  This is particularly useful when invoking a script using the
      #! construct which only allows one argument.  Example:

       #!/usr/bin/perl -spi.bak # same as -s -p -i.bak
       ...

      Options include:

      -0digits
       specifies the record separator ($/) as an octal number.  If there
       are no digits, the null character is the separator.    Other
       switches may precede or follow the digits.  For example, if you
       have a version of find which can print filenames terminated by
       the null character, you can say this:

           find . -name '*.bak' -print0 | perl -n0e unlink

       The special value 00 will cause Perl to slurp files in paragraph
       mode.  The value 0777 will cause Perl to slurp files whole since
       there is no legal character with that value.

      -a   turns on autosplit mode when used with a -n or -p.  An implicit
       split command to the @F array is done as the first thing inside
       the implicit while loop produced by the -n or -p.

        perl -ane 'print pop(@F), "\n";'

       is equivalent to

        while (<>) {
             @F = split(' ');
             print pop(@F), "\n";
        }


      -c   causes perl to check the syntax of the script and then exit
       without executing it.

      -d   runs the script under the perl debugger.  See the section on
       Debugging.

      -Dnumber
       sets debugging flags.  To watch how it executes your script, use
       -D14.  (This only works if debugging is compiled into your perl.)
       Another nice value is -D1024, which lists your compiled syntax
       tree.  And -D512 displays compiled regular expressions.

      -e commandline
       may be used to enter one line of script.  Multiple -e commands
       may be given to build up a multi-line script.  If -e is given,
       perl will not look for a script filename in the argument list.

      -iextension
       specifies that files processed by the <> construct are to be
       edited in-place.  It does this by renaming the input file,
       opening the output file by the same name, and selecting that
       output file as the default for print statements.  The extension,
       if supplied, is added to the name of the old file to make a
       backup copy.  If no extension is supplied, no backup is made.
       Saying "perl -p -i.bak -e "s/foo/bar/;" ... " is the same as
       using the script:

        #!/usr/bin/perl -pi.bak
        s/foo/bar/;

       which is equivalent to

        #!/usr/bin/perl
        while (<>) {
             if ($ARGV ne $oldargv) {
              rename($ARGV, $ARGV . '.bak');
              open(ARGVOUT, ">$ARGV");
              select(ARGVOUT);
              $oldargv = $ARGV;
             }
             s/foo/bar/;
        }
        continue {
            print;     # this prints to original filename
        }
        select(STDOUT);

       except that the -i form doesn't need to compare $ARGV to $oldargv
       to know when the filename has changed.  It does, however, use
       ARGVOUT for the selected filehandle.  Note that STDOUT is
       restored as the default output filehandle after the loop.

       You can use eof to locate the end of each input file, in case you
       want to append to each file, or reset line numbering (see example
       under eof).

      -Idirectory
       may be used in conjunction with -P to tell the C preprocessor
       where to look for include files.  By default /usr/include and
       /usr/lib/perl are searched.

      -loctnum
       enables automatic line-ending processing.  It has two effects:
       first, it automatically chops the line terminator when used with
       -n or -p , and second, it assigns $\ to have the value of octnum
       so that any print statements will have that line terminator added
       back on.  If octnum is omitted, sets $\ to the current value of
       $/.    For instance, to trim lines to 80 columns:

        perl -lpe 'substr($_, 80) = ""'

       Note that the assignment $\ = $/ is done when the switch is
       processed, so the input record separator can be different than
       the output record separator if the -l switch is followed by a -0
       switch:

        gnufind / -print0 | perl -ln0e 'print "found $_" if -p'

       This sets $\ to newline and then sets $/ to the null character.

      -n   causes perl to assume the following loop around your script,
       which makes it iterate over filename arguments somewhat like "sed
       -n" or awk:

        while (<>) {
             ...       # your script goes here
        }

       Note that the lines are not printed by default.  See -p to have
       lines printed.  Here is an efficient way to delete all files
       older than a week:

        find . -mtime +7 -print | perl -nle 'unlink;'

       This is faster than using the -exec switch of find because you
       don't have to start a process on every filename found.

      -p   causes perl to assume the following loop around your script,
       which makes it iterate over filename arguments somewhat like sed:

        while (<>) {
             ...       # your script goes here
        } continue {
             print;
        }

       Note that the lines are printed automatically.  To suppress
       printing use the -n switch.    A -p overrides a -n switch.

      -P   causes your script to be run through the C preprocessor before
       compilation by perl.  (Since both comments and cpp directives
       begin with the # character, you should avoid starting comments
       with any words recognized by the C preprocessor such as "if",
       "else" or "define".)

      -s   enables some rudimentary switch parsing for switches on the
       command line after the script name but before any filename
       arguments (or before a --).    Any switch found there is removed
       from @ARGV and sets the corresponding variable in the perl
       script.  The following script prints "true" if and only if the
       script is invoked with a -xyz switch.

        #!/usr/bin/perl -s
        if ($xyz) { print "true\n"; }


      -S   makes perl use the PATH environment variable to search for the
       script (unless the name of the script starts with a slash).
       Typically this is used to emulate #! startup on machines that
       don't support #!, in the following manner:

        #!/usr/bin/perl
        eval "exec /usr/bin/perl -S $0 $*"
             if $running_under_some_shell;

       The system ignores the first line and feeds the script to
       /bin/sh, which proceeds to try to execute the perl script as a
       shell script.  The shell executes the second line as a normal
       shell command, and thus starts up the perl interpreter.  On some
       systems $0 doesn't always contain the full pathname, so the -S
       tells perl to search for the script if necessary.  After perl
       locates the script, it parses the lines and ignores them because
       the variable $running_under_some_shell is never true.  A better
       construct than $* would be ${1+"$@"}, which handles embedded
       spaces and such in the filenames, but doesn't work if the script
       is being interpreted by csh.  In order to start up sh rather than
       csh, some systems may have to replace the #! line with a line
       containing just a colon, which will be politely ignored by perl.
       Other systems can't control that, and need a totally devious
       construct that will work under any of csh, sh or perl, such as
       the following:

        eval '(exit $?0)' && eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
        & eval 'exec /usr/bin/perl -S $0 $argv:q'
             if 0;


      -u   causes perl to dump core after compiling your script.  You can
       then take this core dump and turn it into an executable file by
       using the undump program (not supplied).  This speeds startup at
       the expense of some disk space (which you can minimize by
       stripping the executable).  (Still, a "hello world" executable
       comes out to about 200K on my machine.) If you are going to run
       your executable as a set-id program then you should probably
       compile it using taintperl rather than normal perl.    If you want
       to execute a portion of your script before dumping, use the dump
       operator instead.  Note: availability of undump is platform
       specific and may not be available for a specific port of perl.

      -U   allows perl to do unsafe operations.  Currently the only "unsafe"
       operations are the unlinking of directories while running as
       superuser, and running setuid programs with fatal taint checks
       turned into warnings.

      -v   prints the version and patchlevel of your perl executable.

      -w   prints warnings about identifiers that are mentioned only once,
       and scalar variables that are used before being set.  Also warns
       about redefined subroutines, and references to undefined
       filehandles or filehandles opened readonly that you are
       attempting to write on.  Also warns you if you use == on values
       that don't look like numbers, and if your subroutines recurse
       more than 100 deep.

      -xdirectory
       tells perl that the script is embedded in a message.  Leading
       garbage will be discarded until the first line that starts with
       #! and contains the string "perl".  Any meaningful switches on
       that line will be applied (but only one group of switches, as
       with normal #! processing).    If a directory name is specified,
       Perl will switch to that directory before running the script.
       The -x switch only controls the the disposal of leading garbage.
       The script must be terminated with __END__ if there is trailing
       garbage to be ignored (the script can process any or all of the
       trailing garbage via the DATA filehandle if desired).

      Data Types and Objects

      Perl has three data types: scalars, arrays of scalars, and associative
      arrays of scalars.  Normal arrays are indexed by number, and
      associative arrays by string.

      The interpretation of operations and values in perl sometimes depends
      on the requirements of the context around the operation or value.
      There are three major contexts: string, numeric and array.  Certain
      operations return array values in contexts wanting an array, and
      scalar values otherwise.    (If this is true of an operation it will be
      mentioned in the documentation for that operation.) Operations which
      return scalars don't care whether the context is looking for a string
      or a number, but scalar variables and values are interpreted as
      strings or numbers as appropriate to the context.  A scalar is
      interpreted as TRUE in the boolean sense if it is not the null string
      or 0.  Booleans returned by operators are 1 for true and 0 or '' (the
      null string) for false.

      There are actually two varieties of null string: defined and
      undefined.  Undefined null strings are returned when there is no real
      value for something, such as when there was an error, or at end of
      file, or when you refer to an uninitialized variable or element of an
      array.  An undefined null string may become defined the first time you
      access it, but prior to that you can use the defined() operator to
      determine whether the value is defined or not.

      References to scalar variables always begin with '$', even when
      referring to a scalar that is part of an array.  Thus:

      $days       # a simple scalar variable
      $days[28]      # 29th element of array @days
      $days{'Feb'}      # one value from an associative array
      $#days      # last index of array @days

      but entire arrays or array slices are denoted by '@':

      @days       # ($days[0], $days[1],... $days[n])
      @days[3,4,5]      # same as @days[3..5]
      @days{'a','c'}  # same as ($days{'a'},$days{'c'})

      and entire associative arrays are denoted by '%':

      %days       # (key1, val1, key2, val2 ...)

      Any of these eight constructs may serve as an lvalue, that is, may be
      assigned to.  (It also turns out that an assignment is itself an
      lvalue in certain contexts--see examples under s, tr and chop.)
      Assignment to a scalar evaluates the righthand side in a scalar
      context, while assignment to an array or array slice evaluates the
      righthand side in an array context.

      You may find the length of array @days by evaluating "$#days", as in
      csh.  (Actually, it's not the length of the array, it's the subscript
      of the last element, since there is (ordinarily) a 0th element.)
      Assigning to $#days changes the length of the array.  Shortening an
      array by this method does not actually destroy any values.
      Lengthening an array that was previously shortened recovers the values
      that were in those elements.  You can also gain some measure of
      efficiency by preextending an array that is going to get big.  (You
      can also extend an array by assigning to an element that is off the
      end of the array.  This differs from assigning to $#whatever in that
      intervening values are set to null rather than recovered.) You can
      truncate an array down to nothing by assigning the null list () to it.
      The following are exactly equivalent

       @whatever = ();
       $#whatever = $[ - 1;

      If you evaluate an array in a scalar context, it returns the length of
      the array.  The following is always true:

       scalar(@whatever) == $#whatever - $[ + 1;

      If you evaluate an associative array in a scalar context, it returns a
      value which is true if and only if the array contains any elements.
      (If there are any elements, the value returned is a string consisting
      of the number of used buckets and the number of allocated buckets,
      separated by a slash.)

      Multi-dimensional arrays are not directly supported, but see the
      discussion of the $; variable later for a means of emulating multiple
      subscripts with an associative array.  You could also write a
      subroutine to turn multiple subscripts into a single subscript.

      Every data type has its own namespace.  You can, without fear of
      conflict, use the same name for a scalar variable, an array, an
      associative array, a filehandle, a subroutine name, and/or a label.
      Since variable and array references always start with '$', '@', or
      '%', the "reserved" words aren't in fact reserved with respect to
      variable names.  (They ARE reserved with respect to labels and
      filehandles, however, which don't have an initial special character.
      Hint: you could say open(LOG,'logfile') rather than
      open(log,'logfile').  Using uppercase filehandles also improves
      readability and protects you from conflict with future reserved
      words.) Case IS significant--"FOO", "Foo" and "foo" are all different
      names.  Names which start with a letter may also contain digits and
      underscores.  Names which do not start with a letter are limited to
      one character, e.g. "$%" or "$$".  (Most of the one character names
      have a predefined significance to perl.  More later.)

      Numeric literals are specified in any of the usual floating point or
      integer formats:

      12345
      12345.67
      .23E-10
      0xffff     # hex
      0377    # octal
      4_294_967_296

      String literals are delimited by either single or double quotes.    They
      work much like shell quotes: double-quoted string literals are subject
      to backslash and variable substitution; single-quoted strings are not
      (except for \' and \\).  The usual backslash rules apply for making
      characters such as newline, tab, etc., as well as some more exotic
      forms:

       \t         tab
       \n         newline
       \r         return
       \f         form feed
       \b         backspace
       \a         alarm (bell)
       \e         escape
       \033      octal char
       \x1b      hex char
       \c[         control char
       \l         lowercase next char
       \u         uppercase next char
       \L         lowercase till \E
       \U         uppercase till \E
       \E         end case modification

      You can also embed newlines directly in your strings, i.e. they can
      end on a different line than they begin.    This is nice, but if you
      forget your trailing quote, the error will not be reported until perl
      finds another line containing the quote character, which may be much
      further on in the script.  Variable substitution inside strings is
      limited to scalar variables, normal array values, and array slices.
      (In other words, identifiers beginning with $ or @, followed by an
      optional bracketed expression as a subscript.) The following code
      segment prints out "The price is $100."

      $Price = '$100';         # not interpreted
      print "The price is $Price.\n";# interpreted

      Note that you can put curly brackets around the identifier to delimit
      it from following alphanumerics.    Also note that a single quoted
      string must be separated from a preceding word by a space, since
      single quote is a valid character in an identifier (see Packages).

      Two special literals are __LINE__ and __FILE__, which represent the
      current line number and filename at that point in your program.  They
      may only be used as separate tokens; they will not be interpolated
      into strings.  In addition, the token __END__ may be used to indicate
      the logical end of the script before the actual end of file.  Any
      following text is ignored, but may be read via the DATA filehandle.
      (The DATA filehandle may read data only from the main script, but not
      from any required file or evaluated string.) The two control
      characters ^D and ^Z are synonyms for __END__.

      A word that doesn't have any other interpretation in the grammar will
      be treated as if it had single quotes around it.    For this purpose, a
      word consists only of alphanumeric characters and underline, and must
      start with an alphabetic character.  As with filehandles and labels, a
      bare word that consists entirely of lowercase letters risks conflict
      with future reserved words, and if you use the -w switch, Perl will
      warn you about any such words.

      Array values are interpolated into double-quoted strings by joining
      all the elements of the array with the delimiter specified in the $"
      variable, space by default.  (Since in versions of perl prior to 3.0
      the @ character was not a metacharacter in double-quoted strings, the
      interpolation of @array, $array[EXPR], @array[LIST], $array{EXPR}, or
      @array{LIST} only happens if array is referenced elsewhere in the
      program or is predefined.) The following are equivalent:

       $temp = join($",@ARGV);
       system "echo $temp";

       system "echo @ARGV";

      Within search patterns (which also undergo double-quotish
      substitution) there is a bad ambiguity:  Is /$foo[bar]/ to be
      interpreted as /${foo}[bar]/ (where [bar] is a character class for the
      regular expression) or as /${foo[bar]}/ (where [bar] is the subscript
      to array @foo)?  If @foo doesn't otherwise exist, then it's obviously
      a character class.  If @foo exists, perl takes a good guess about
      [bar], and is almost always right.  If it does guess wrong, or if
      you're just plain paranoid, you can force the correct interpretation
      with curly brackets as above.

      A line-oriented form of quoting is based on the shell here-is syntax.
      Following a << you specify a string to terminate the quoted material,
      and all lines following the current line down to the terminating
      string are the value of the item.  The terminating string may be
      either an identifier (a word), or some quoted text.  If quoted, the
      type of quotes you use determines the treatment of the text, just as
      in regular quoting.  An unquoted identifier works like double quotes.
      There must be no space between the << and the identifier.  (If you put
      a space it will be treated as a null identifier, which is valid, and
      matches the first blank line--see Merry Christmas example below.) The
      terminating string must appear by itself (unquoted and with no
      surrounding whitespace) on the terminating line.

       print <<EOF;        # same as above
      The price is $Price.
      EOF

       print <<"EOF";      # same as above
      The price is $Price.
      EOF

       print << x 10;      # null identifier is delimiter
      Merry Christmas!

       print <<`EOC`;      # execute commands
      echo hi there
      echo lo there
      EOC

       print <<foo, <<bar; # you can stack them
      I said foo.
      foo
      I said bar.
      bar

      Array literals are denoted by separating individual values by commas,
      and enclosing the list in parentheses:

       (LIST)

      In a context not requiring an array value, the value of the array
      literal is the value of the final element, as in the C comma operator.
      For example,

      @foo = ('cc', '-E', $bar);

      assigns the entire array value to array foo, but

      $foo = ('cc', '-E', $bar);

      assigns the value of variable bar to variable foo.  Note that the
      value of an actual array in a scalar context is the length of the
      array; the following assigns to $foo the value 3:

      @foo = ('cc', '-E', $bar);
      $foo = @foo;           # $foo gets 3

      You may have an optional comma before the closing parenthesis of an
      array literal, so that you can say:

      @foo = (
       1,
       2,
       3,
      );

      When a LIST is evaluated, each element of the list is evaluated in an
      array context, and the resulting array value is interpolated into LIST
      just as if each individual element were a member of LIST.  Thus arrays
      lose their identity in a LIST--the list

       (@foo,@bar,&SomeSub)

      contains all the elements of @foo followed by all the elements of
      @bar, followed by all the elements returned by the subroutine named
      SomeSub.

      A list value may also be subscripted like a normal array.  Examples:

       $time = (stat($file))[8];     # stat returns array value
       $digit = ('a','b','c','d','e','f')[$digit-10];
       return (pop(@foo),pop(@foo))[0];


      Array lists may be assigned to if and only if each element of the list
      is an lvalue:

      ($a, $b, $c) = (1, 2, 3);

      ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);

      The final element may be an array or an associative array:

      ($a, $b, @rest) = split;
      local($a, $b, %rest) = @_;

      You can actually put an array anywhere in the list, but the first
      array in the list will soak up all the values, and anything after it
      will get a null value.  This may be useful in a local().

      An associative array literal contains pairs of values to be
      interpreted as a key and a value:

      # same as map assignment above
      %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);

      Array assignment in a scalar context returns the number of elements
      produced by the expression on the right side of the assignment:

       $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2


      There are several other pseudo-literals that you should know about.
      If a string is enclosed by backticks (grave accents), it first
      undergoes variable substitution just like a double quoted string.  It
      is then interpreted as a command, and the output of that command is
      the value of the pseudo-literal, like in a shell.  In a scalar
      context, a single string consisting of all the output is returned.  In
      an array context, an array of values is returned, one for each line of
      output.  (You can set $/ to use a different line terminator.) The
      command is executed each time the pseudo-literal is evaluated.  The
      status value of the command is returned in $? (see Predefined Names
      for the interpretation of $?).  Unlike in csh, no translation is done
      on the return data--newlines remain newlines.  Unlike in any of the
      shells, single quotes do not hide variable names in the command from
      interpretation.  To pass a $ through to the shell you need to hide it
      with a backslash.

      Evaluating a filehandle in angle brackets yields the next line from
      that file (newline included, so it's never false until EOF, at which
      time an undefined value is returned).  Ordinarily you must assign that
      value to a variable, but there is one situation where an automatic
      assignment happens.  If (and only if) the input symbol is the only
      thing inside the conditional of a while loop, the value is
      automatically assigned to the variable "$_".  (This may seem like an
      odd thing to you, but you'll use the construct in almost every perl
      script you write.) Anyway, the following lines are equivalent to each
      other:

      while ($_ = <STDIN>) { print; }
      while (<STDIN>) { print; }
      for (;<STDIN>;) { print; }
      print while $_ = <STDIN>;
      print while <STDIN>;

      The filehandles STDIN, STDOUT and STDERR are predefined.    (The
      filehandles stdin, stdout and stderr will also work except in
      packages, where they would be interpreted as local identifiers rather
      than global.) Additional filehandles may be created with the open
      function.

      If a <FILEHANDLE> is used in a context that is looking for an array,
      an array consisting of all the input lines is returned, one line per
      array element.  It's easy to make a LARGE data space this way, so use
      with care.

      The null filehandle <> is special and can be used to emulate the
      behavior of sed and awk.    Input from <> comes either from standard
      input, or from each file listed on the command line.  Here's how it
      works: the first time <> is evaluated, the ARGV array is checked, and
      if it is null, $ARGV[0] is set to '-', which when opened gives you
      standard input.  The ARGV array is then processed as a list of
      filenames.  The loop

       while (<>) {
        ...           # code for each line
       }

      is equivalent to the following Perl-like pseudo code:

       unshift(@ARGV, '-') if $#ARGV < $[;
       while ($ARGV = shift) {
        open(ARGV, $ARGV);
        while (<ARGV>) {
             ...       # code for each line
        }
       }

      except that it isn't as cumbersome to say, and will actually work.  It
      really does shift array ARGV and put the current filename into
      variable ARGV.  It also uses filehandle ARGV internally--<> is just a
      synonym for <ARGV>, which is magical.  (The pseudo code above doesn't
      work because it treats <ARGV> as non-magical.)

      You can modify @ARGV before the first <> as long as the array ends up
      containing the list of filenames you really want.  Line numbers ($.)
      continue as if the input was one big happy file.    (But see example
      under eof for how to reset line numbers on each file.)

      If you want to set @ARGV to your own list of files, go right ahead.
      If you want to pass switches into your script, you can put a loop on
      the front like this:

       while ($_ = $ARGV[0], /^-/) {
        shift;
           last if /^--$/;
        /^-D(.*)/ && ($debug = $1);
        /^-v/ && $verbose++;
        ...      # other switches
       }
       while (<>) {
        ...      # code for each line
       }

      The <> symbol will return FALSE only once.  If you call it again after
      this it will assume you are processing another @ARGV list, and if you
      haven't set @ARGV, will input from STDIN.

      If the string inside the angle brackets is a reference to a scalar
      variable (e.g. <$foo>), then that variable contains the name of the
      filehandle to input from.

      If the string inside angle brackets is not a filehandle, it is
      interpreted as a filename pattern to be globbed, and either an array
      of filenames or the next filename in the list is returned, depending
      on context.  One level of $ interpretation is done first, but you
      can't say <$foo> because that's an indirect filehandle as explained in
      the previous paragraph.  You could insert curly brackets to force
      interpretation as a filename glob: <${foo}>.  Example:

       while (<*.c>) {
        chmod 0644, $_;
       }

      is equivalent to

       open(foo, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
       while (<foo>) {
        chop;
        chmod 0644, $_;
       }

      In fact, it's currently implemented that way.  (Which means it will
      not work on filenames with spaces in them unless you have /bin/csh on
      your machine.) Of course, the shortest way to do the above is:

       chmod 0644, <*.c>;


      Syntax

      A perl script consists of a sequence of declarations and commands.
      The only things that need to be declared in perl are report formats
      and subroutines.    See the sections below for more information on those
      declarations.  All uninitialized user-created objects are assumed to
      start with a null or 0 value until they are defined by some explicit
      operation such as assignment.  The sequence of commands is executed
      just once, unlike in sed and awk scripts, where the sequence of
      commands is executed for each input line.  While this means that you
      must explicitly loop over the lines of your input file (or files), it
      also means you have much more control over which files and which lines
      you look at.  (Actually, I'm lying--it is possible to do an implicit
      loop with either the -n or -p switch.)

      A declaration can be put anywhere a command can, but has no effect on
      the execution of the primary sequence of commands--declarations all
      take effect at compile time.  Typically all the declarations are put
      at the beginning or the end of the script.

      Perl is, for the most part, a free-form language.  (The only exception
      to this is format declarations, for fairly obvious reasons.) Comments
      are indicated by the # character, and extend to the end of the line.
      If you attempt to use /* */ C comments, it will be interpreted either
      as division or pattern matching, depending on the context.  So don't
      do that.

      Compound statements

      In perl, a sequence of commands may be treated as one command by
      enclosing it in curly brackets.  We will call this a BLOCK.

      The following compound commands may be used to control flow:

       if (EXPR) BLOCK
       if (EXPR) BLOCK else BLOCK
       if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
       LABEL while (EXPR) BLOCK
       LABEL while (EXPR) BLOCK continue BLOCK
       LABEL for (EXPR; EXPR; EXPR) BLOCK
       LABEL foreach VAR (ARRAY) BLOCK
       LABEL BLOCK continue BLOCK

      Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
      not statements.  This means that the curly brackets are required--no
      dangling statements allowed.  If you want to write conditionals
      without curly brackets there are several other ways to do it.  The
      following all do the same thing:

       if (!open(foo)) { die "Can't open $foo: $!"; }
       die "Can't open $foo: $!" unless open(foo);
       open(foo) || die "Can't open $foo: $!"; # foo or bust!
       open(foo) ? 'hi mom' : die "Can't open $foo: $!";
              # a bit exotic, that last one


      The if statement is straightforward.  Since BLOCKs are always bounded
      by curly brackets, there is never any ambiguity about which if an else
      goes with.  If you use unless in place of if, the sense of the test is
      reversed.

      The while statement executes the block as long as the expression is
      true (does not evaluate to the null string or 0).  The LABEL is
      optional, and if present, consists of an identifier followed by a
      colon.  The LABEL identifies the loop for the loop control statements
      next, last, and redo (see below).  If there is a continue BLOCK, it is
      always executed just before the conditional is about to be evaluated
      again, similarly to the third part of a for loop in C.  Thus it can be
      used to increment a loop variable, even when the loop has been
      continued via the next statement (similar to the C "continue"
      statement).

      If the word while is replaced by the word until, the sense of the test
      is reversed, but the conditional is still tested before the first
      iteration.

      In either the if or the while statement, you may replace "(EXPR)" with
      a BLOCK, and the conditional is true if the value of the last command
      in that block is true.

      The for loop works exactly like the corresponding while loop:

       for ($i = 1; $i < 10; $i++) {
        ...
       }

      is the same as

       $i = 1;
       while ($i < 10) {
        ...
       } continue {
        $i++;
       }

      The foreach loop iterates over a normal array value and sets the
      variable VAR to be each element of the array in turn.  The variable is
      implicitly local to the loop, and regains its former value upon
      exiting the loop.  The "foreach" keyword is actually identical to the
      "for" keyword, so you can use "foreach" for readability or "for" for
      brevity.    If VAR is omitted, $_ is set to each value.  If ARRAY is an
      actual array (as opposed to an expression returning an array value),
      you can modify each element of the array by modifying VAR inside the
      loop.  Examples:

       for (@ary) { s/foo/bar/; }

       foreach $elem (@elements) {
        $elem *= 2;
       }

       for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) {
        print $_, "\n"; sleep(1);
       }

       for (1..15) { print "Merry Christmas\n"; }

       foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})) {
        print "Item: $item\n";
       }


      The BLOCK by itself (labeled or not) is equivalent to a loop that
      executes once.  Thus you can use any of the loop control statements in
      it to leave or restart the block.  The continue block is optional.
      This construct is particularly nice for doing case structures.

       foo: {
        if (/^abc/) { $abc = 1; last foo; }
        if (/^def/) { $def = 1; last foo; }
        if (/^xyz/) { $xyz = 1; last foo; }
        $nothing = 1;
       }

      There is no official switch statement in perl, because there are
      already several ways to write the equivalent.  In addition to the
      above, you could write

       foo: {
        $abc = 1, last foo  if /^abc/;
        $def = 1, last foo  if /^def/;
        $xyz = 1, last foo  if /^xyz/;
        $nothing = 1;
       }

      or

       foo: {
        /^abc/ && do { $abc = 1; last foo; };
        /^def/ && do { $def = 1; last foo; };
        /^xyz/ && do { $xyz = 1; last foo; };
        $nothing = 1;
       }

      or

       foo: {
        /^abc/ && ($abc = 1, last foo);
        /^def/ && ($def = 1, last foo);
        /^xyz/ && ($xyz = 1, last foo);
        $nothing = 1;
       }

      or even

       if (/^abc/)
        { $abc = 1; }
       elsif (/^def/)
        { $def = 1; }
       elsif (/^xyz/)
        { $xyz = 1; }
       else
        {$nothing = 1;}

      As it happens, these are all optimized internally to a switch
      structure, so perl jumps directly to the desired statement, and you
      needn't worry about perl executing a lot of unnecessary statements
      when you have a string of 50 elsifs, as long as you are testing the
      same simple scalar variable using ==, eq, or pattern matching as
      above.  (If you're curious as to whether the optimizer has done this
      for a particular case statement, you can use the -D1024 switch to list
      the syntax tree before execution.)

      Simple statements

      The only kind of simple statement is an expression evaluated for its
      side effects.  Every simple statement must be terminated with a
      semicolon, unless it is the final statement in a block, in which case
      the semicolon is optional.  (Semicolon is still encouraged there if
      the block takes up more than one line).

      Any simple statement may optionally be followed by a single modifier,
      just before the terminating semicolon.  The possible modifiers are:

       if EXPR
       unless EXPR
       while EXPR
       until EXPR

      The if and unless modifiers have the expected semantics.    The while
      and until modifiers also have the expected semantics (conditional
      evaluated first), except when applied to a do-BLOCK or a do-SUBROUTINE
      command, in which case the block executes once before the conditional
      is evaluated.  This is so that you can write loops like:

       do {
        $_ = <STDIN>;
        ...
       } until $_ eq ".\n";

      (See the do operator below.  Note also that the loop control commands
      described later will NOT work in this construct, since modifiers don't
      take loop labels.  Sorry.)

      Expressions

      Since perl expressions work almost exactly like C expressions, only
      the differences will be mentioned here.

      Here's what perl has that C doesn't:

      **      The exponentiation operator.

      **=     The exponentiation assignment operator.

      ()      The null list, used to initialize an array to null.

      .       Concatenation of two strings.

      .=      The concatenation assignment operator.

      eq      String equality (== is numeric equality).  For a mnemonic just
          think of "eq" as a string.  (If you are used to the awk
          behavior of using == for either string or numeric equality
          based on the current form of the comparands, beware!  You must
          be explicit here.)

      ne      String inequality (!= is numeric inequality).

      lt      String less than.

      gt      String greater than.

      le      String less than or equal.

      ge      String greater than or equal.

      cmp     String comparison, returning -1, 0, or 1.

      <=>     Numeric comparison, returning -1, 0, or 1.

      =~      Certain operations search or modify the string "$_" by
          default.    This operator makes that kind of operation work on
          some other string.  The right argument is a search pattern,
          substitution, or translation.  The left argument is what is
          supposed to be searched, substituted, or translated instead of
          the default "$_".  The return value indicates the success of
          the operation.  (If the right argument is an expression other
          than a search pattern, substitution, or translation, it is
          interpreted as a search pattern at run time.  This is less
          efficient than an explicit search, since the pattern must be
          compiled every time the expression is evaluated.) The
          precedence of this operator is lower than unary minus and
          autoincrement/decrement, but higher than everything else.

      !~      Just like =~ except the return value is negated.

      x       The repetition operator.    Returns a string consisting of the
          left operand repeated the number of times specified by the
          right operand.  In an array context, if the left operand is a
          list in parens, it repeats the list.

           print '-' x 80;        # print row of dashes
           print '-' x80;      # illegal, x80 is identifier

           print "\t" x ($tab/8), ' ' x ($tab%8);  # tab over

           @ones = (1) x 80;        # an array of 80 1's
           @ones = (5) x @ones;      # set all elements to 5


      x=      The repetition assignment operator.  Only works on scalars.

      ..      The range operator, which is really two different operators
          depending on the context.  In an array context, returns an
          array of values counting (by ones) from the left value to the
          right value.  This is useful for writing "for (1..10)" loops
          and for doing slice operations on arrays.

          In a scalar context, .. returns a boolean value.    The operator
          is bistable, like a flip-flop, and emulates the line-range
          (comma) operator of sed, awk, and various editors.  Each ..
          operator maintains its own boolean state.  It is false as long
          as its left operand is false.  Once the left operand is true,
          the range operator stays true until the right operand is true,
          AFTER which the range operator becomes false again.  (It
          doesn't become false till the next time the range operator is
          evaluated.  It can test the right operand and become false on
          the same evaluation it became true (as in awk), but it still
          returns true once.  If you don't want it to test the right
          operand till the next evaluation (as in sed), use three dots
          (...) instead of two.) The right operand is not evaluated
          while the operator is in the "false" state, and the left
          operand is not evaluated while the operator is in the "true"
          state.  The precedence is a little lower than || and &&.    The
          value returned is either the null string for false, or a
          sequence number (beginning with 1) for true.  The sequence
          number is reset for each range encountered.  The final
          sequence number in a range has the string 'E0' appended to it,
          which doesn't affect its numeric value, but gives you
          something to search for if you want to exclude the endpoint.
          You can exclude the beginning point by waiting for the
          sequence number to be greater than 1.  If either operand of
          scalar .. is static, that operand is implicitly compared to
          the $. variable, the current line number.  Examples:

          As a scalar operator:
          if (101 .. 200) { print; }     # print 2nd hundred lines

          next line if (1 .. /^$/); # skip header lines

          s/^/> / if (/^$/ .. eof());     # quote body

          As an array operator:
          for (101 .. 200) { print; }     # print $_ 100 times

          @foo = @foo[$[ .. $#foo]; # an expensive no-op
          @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items


      -x      A file test.  This unary operator takes one argument, either a
          filename or a filehandle, and tests the associated file to see
          if something is true about it.  If the argument is omitted,
          tests $_, except for -t, which tests STDIN.  It returns 1 for
          true and '' for false, or the undefined value if the file
          doesn't exist.  Precedence is higher than logical and
          relational operators, but lower than arithmetic operators.
          The operator may be any of:
           -r    File is readable by effective uid/gid.
           -w    File is writable by effective uid/gid.
           -x    File is executable by effective uid/gid.
           -o    File is owned by effective uid.
           -R    File is readable by real uid/gid.
           -W    File is writable by real uid/gid.
           -X    File is executable by real uid/gid.
           -O    File is owned by real uid.
           -e    File exists.
           -z    File has zero size.
           -s    File has non-zero size (returns size).
           -f    File is a plain file.
           -d    File is a directory.
           -l    File is a symbolic link.
           -p    File is a named pipe (FIFO).
           -S    File is a socket.
           -b    File is a block special file.
           -c    File is a character special file.
           -u    File has setuid bit set.
           -g    File has setgid bit set.
           -k    File has sticky bit set.
           -t    Filehandle is opened to a tty.
           -T    File is a text file.
           -B    File is a binary file (opposite of -T).
           -M    Age of file in days when script started.
           -A    Same for access time.
           -C    Same for inode change time.

          The interpretation of the file permission operators -r, -R,
          -w, -W, -x and -X is based solely on the mode of the file and
          the uids and gids of the user.  There may be other reasons you
          can't actually read, write or execute the file.  Also note
          that, for the superuser, -r, -R, -w and -W always return 1,
          and -x and -X return 1 if any execute bit is set in the mode.
          Scripts run by the superuser may thus need to do a stat() in
          order to determine the actual mode of the file, or temporarily
          set the uid to something else.

          Example:

           while (<>) {
            chop;
            next unless -f $_;  # ignore specials
            ...
           }

          Note that -s/a/b/ does not do a negated substitution.  Saying
          -exp($foo) still works as expected, however--only single
          letters following a minus are interpreted as file tests.

          The -T and -B switches work as follows.  The first block or so
          of the file is examined for odd characters such as strange
          control codes or metacharacters.    If too many odd characters
          (>10%) are found, it's a -B file, otherwise it's a -T file.
          Also, any file containing null in the first block is
          considered a binary file.  If -T or -B is used on a
          filehandle, the current stdio buffer is examined rather than
          the first block.    Both -T and -B return TRUE on a null file,
          or a file at EOF when testing a filehandle.

      If any of the file tests (or either stat operator) are given the
      special filehandle consisting of a solitary underline, then the stat
      structure of the previous file test (or stat operator) is used, saving
      a system call.  (This doesn't work with -t, and you need to remember
      that lstat and -l will leave values in the stat structure for the
      symbolic link, not the real file.) Example:

       print "Can do.\n" if -r $a || -w _ || -x _;

       stat($filename);
       print "Readable\n" if -r _;
       print "Writable\n" if -w _;
       print "Executable\n" if -x _;
       print "Setuid\n" if -u _;
       print "Setgid\n" if -g _;
       print "Sticky\n" if -k _;
       print "Text\n" if -T _;
       print "Binary\n" if -B _;


      Here is what C has that perl doesn't:

      unary &      Address-of operator.

      unary *      Dereference-address operator.

      (TYPE)      Type casting operator.

      Like C, perl does a certain amount of expression evaluation at compile
      time, whenever it determines that all of the arguments to an operator
      are static and have no side effects.  In particular, string
      concatenation happens at compile time between literals that don't do
      variable substitution.  Backslash interpretation also happens at
      compile time.  You can say

       'Now is the time for all' . "\n" .
       'good men to come to.'

      and this all reduces to one string internally.

      The autoincrement operator has a little extra built-in magic to it.
      If you increment a variable that is numeric, or that has ever been
      used in a numeric context, you get a normal increment.  If, however,
      the variable has only been used in string contexts since it was set,
      and has a value that is not null and matches the pattern
      /^[a-zA-Z]*[0-9]*$/, the increment is done as a string, preserving
      each character within its range, with carry:

       print ++($foo = '99');   # prints '100'
       print ++($foo = 'a0');   # prints 'a1'
       print ++($foo = 'Az');   # prints 'Ba'
       print ++($foo = 'zz');   # prints 'aaa'

      The autodecrement is not magical.

      The range operator (in an array context) makes use of the magical
      autoincrement algorithm if the minimum and maximum are strings.  You
      can say

       @alphabet = ('A' .. 'Z');

      to get all the letters of the alphabet, or

       $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];

      to get a hexadecimal digit, or

       @z2 = ('01' .. '31');  print @z2[$mday];

      to get dates with leading zeros.    (If the final value specified is not
      in the sequence that the magical increment would produce, the sequence
      goes until the next value would be longer than the final value
      specified.)

      The || and && operators differ from C's in that, rather than returning
      0 or 1, they return the last value evaluated. Thus, a portable way to
      find out the home directory might be:

       $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
           (getpwuid($<))[7] || die "You're homeless!\n";


      Along with the literals and variables mentioned earlier, the
      operations in the following section can serve as terms in an
      expression.  Some of these operations take a LIST as an argument.
      Such a list can consist of any combination of scalar arguments or
      array values; the array values will be included in the list as if each
      individual element were interpolated at that point in the list,
      forming a longer single-dimensional array value.    Elements of the LIST
      should be separated by commas.  If an operation is listed both with
      and without parentheses around its arguments, it means you can either
      use it as a unary operator or as a function call.  To use it as a
      function call, the next token on the same line must be a left
      parenthesis.  (There may be intervening white space.) Such a function
      then has highest precedence, as you would expect from a function.  If
      any token other than a left parenthesis follows, then it is a unary
      operator, with a precedence depending only on whether it is a LIST
      operator or not.    LIST operators have lowest precedence.    All other
      unary operators have a precedence greater than relational operators
      but less than arithmetic operators.  See the section on Precedence.

      For operators that can be used in either a scalar or array context,
      failure is generally indicated in a scalar context by returning the
      undefined value, and in an array context by returning the null list.
      Remember though that THERE IS NO GENERAL RULE FOR CONVERTING A LIST
      INTO A SCALAR.  Each operator decides which sort of scalar it would be
      most appropriate to return.  Some operators return the length of the
      list that would have been returned in an array context.  Some
      operators return the first value in the list.  Some operators return
      the last value in the list.  Some operators return a count of
      successful operations.  In general, they do what you want, unless you
      want consistency.

      /PATTERN/
          See m/PATTERN/.

      ?PATTERN?
          This is just like the /pattern/ search, except that it matches
          only once between calls to the reset operator.  This is a
          useful optimization when you only want to see the first
          occurrence of something in each file of a set of files, for
          instance.  Only ?? patterns local to the current package are
          reset.

      accept(NEWSOCKET,GENERICSOCKET)
          Does the same thing that the accept system call does.  Returns
          true if it succeeded, false otherwise.  See example in section
          on Interprocess Communication.

      alarm(SECONDS)

      alarm SECONDS
          Arranges to have a SIGALRM delivered to this process after the
          specified number of seconds (minus 1, actually) have elapsed.
          Thus, alarm(15) will cause a SIGALRM at some point more than
          14 seconds in the future.  Only one timer may be counting at
          once.  Each call disables the previous timer, and an argument
          of 0 may be supplied to cancel the previous timer without
          starting a new one.  The returned value is the amount of time
          remaining on the previous timer.

      atan2(Y,X)
          Returns the arctangent of Y/X in the range -PI to PI.

      bind(SOCKET,NAME)
          Does the same thing that the bind system call does.  Returns
          true if it succeeded, false otherwise.  NAME should be a
          packed address of the proper type for the socket.  See example
          in section on Interprocess Communication.

      binmode(FILEHANDLE)

      binmode FILEHANDLE
          Arranges for the file to be read in "binary" mode in operating
          systems that distinguish between binary and text files.  Files
          that are not read in binary mode have CR LF sequences
          translated to LF on input and LF translated to CR LF on
          output.  Binmode has no effect under Unix.  If FILEHANDLE is
          an expression, the value is taken as the name of the
          filehandle.

      caller(EXPR)

      caller  Returns the context of the current subroutine call:

           ($package,$filename,$line) = caller;

          With EXPR, returns some extra information that the debugger
          uses to print a stack trace.  The value of EXPR indicates how
          many call frames to go back before the current one.

      chdir(EXPR)

      chdir EXPR
          Changes the working directory to EXPR, if possible.  If EXPR
          is omitted, changes to home directory.  Returns 1 upon
          success, 0 otherwise.  See example under die.

      chmod(LIST)

      chmod LIST
          Changes the permissions of a list of files.  The first element
          of the list must be the numerical mode.  Returns the number of
          files successfully changed.

           $cnt = chmod 0755, 'foo', 'bar';
           chmod 0755, @executables;


      chop(LIST)

      chop(VARIABLE)

      chop VARIABLE

      chop    Chops off the last character of a string and returns the
          character chopped.  It's used primarily to remove the newline
          from the end of an input record, but is much more efficient
          than s/\n// because it neither scans nor copies the string.
          If VARIABLE is omitted, chops $_.  Example:

           while (<>) {
            chop;      # avoid \n on last field
            @array = split(/:/);
            ...
           }

          You can actually chop anything that's an lvalue, including an
          assignment:

           chop($cwd = `pwd`);
           chop($answer = <STDIN>);

          If you chop a list, each element is chopped.  Only the value
          of the last chop is returned.

      chown(LIST)

      chown LIST
          Changes the owner (and group) of a list of files.  The first
          two elements of the list must be the NUMERICAL uid and gid, in
          that order.  Returns the number of files successfully changed.

           $cnt = chown $uid, $gid, 'foo', 'bar';
           chown $uid, $gid, @filenames;

          Here's an example that looks up non-numeric uids in the passwd
          file:

           print "User: ";
           $user = <STDIN>;
           chop($user);
           print "Files: "
           $pattern = <STDIN>;
           chop($pattern);
           open(pass, '/etc/passwd')
            || die "Can't open passwd: $!\n";
           while (<pass>) {
            ($login,$pass,$uid,$gid) = split(/:/);
            $uid{$login} = $uid;
            $gid{$login} = $gid;
           }
           @ary = <${pattern}>;     # get filenames
           if ($uid{$user} eq '') {
            die "$user not in passwd file";
           }
           else {
            chown $uid{$user}, $gid{$user}, @ary;
           }

      chroot(FILENAME)

      chroot FILENAME
          Does the same as the system call of that name.  If you don't
          know what it does, don't worry about it.    If FILENAME is
          omitted, does chroot to $_.

      close(FILEHANDLE)

      close FILEHANDLE
          Closes the file or pipe associated with the file handle.    You
          don't have to close FILEHANDLE if you are immediately going to
          do another open on it, since open will close it for you.    (See
          open.) However, an explicit close on an input file resets the
          line counter ($.), while the implicit close done by open does
          not.  Also, closing a pipe will wait for the process executing
          on the pipe to complete, in case you want to look at the
          output of the pipe afterwards.  Closing a pipe explicitly also
          puts the status value of the command into $?.  Example:

           open(OUTPUT, '|sort >foo');     # pipe to sort
           ...    # print stuff to output
           close OUTPUT;       # wait for sort to finish
           open(INPUT, 'foo'); # get sort's results

          FILEHANDLE may be an expression whose value gives the real
          filehandle name.

      closedir(DIRHANDLE)

      closedir DIRHANDLE
          Closes a directory opened by opendir().

      connect(SOCKET,NAME)
          Does the same thing that the connect system call does.
          Returns true if it succeeded, false otherwise.  NAME should be
          a package address of the proper type for the socket.  See
          example in section on Interprocess Communication.

      cos(EXPR)

      cos EXPR
          Returns the cosine of EXPR (expressed in radians).  If EXPR is
          omitted takes cosine of $_.

      crypt(PLAINTEXT,SALT)
          Encrypts a string exactly like the crypt() function in the C
          library.    Useful for checking the password file for lousy
          passwords.  Only the guys wearing white hats should do this.

      dbmclose(ASSOC_ARRAY)

      dbmclose ASSOC_ARRAY
          Breaks the binding between a dbm file and an associative
          array.  The values remaining in the associative array are
          meaningless unless you happen to want to know what was in the
          cache for the dbm file.  This function is only useful if you
          have ndbm.

      dbmopen(ASSOC,DBNAME,MODE)
          This binds a dbm or ndbm file to an associative array.  ASSOC
          is the name of the associative array.  (Unlike normal open,
          the first argument is NOT a filehandle, even though it looks
          like one).  DBNAME is the name of the database (without the
          .dir or .pag extension).    If the database does not exist, it
          is created with protection specified by MODE (as modified by
          the umask).  If your system only supports the older dbm
          functions, you may perform only one dbmopen in your program.
          If your system has neither dbm nor ndbm, calling dbmopen
          produces a fatal error.

          Values assigned to the associative array prior to the dbmopen
          are lost.  A certain number of values from the dbm file are
          cached in memory.  By default this number is 64, but you can
          increase it by preallocating that number of garbage entries in
          the associative array before the dbmopen.  You can flush the
          cache if necessary with the reset command.

          If you don't have write access to the dbm file, you can only
          read associative array variables, not set them.  If you want
          to test whether you can write, either use file tests or try
          setting a dummy array entry inside an eval, which will trap
          the error.

          Note that functions such as keys() and values() may return
          huge array values when used on large dbm files.  You may
          prefer to use the each() function to iterate over large dbm
          files.  Example:

           # print out history file offsets
           dbmopen(HIST,'/usr/lib/news/history',0666);
           while (($key,$val) = each %HIST) {
            print $key, ' = ', unpack('L',$val), "\n";
           }
           dbmclose(HIST);

      defined(EXPR)

      defined EXPR
          Returns a boolean value saying whether the lvalue EXPR has a
          real value or not.  Many operations return the undefined value
          under exceptional conditions, such as end of file,
          uninitialized variable, system error and such.  This function
          allows you to distinguish between an undefined null string and
          a defined null string with operations that might return a real
          null string, in particular referencing elements of an array.
          You may also check to see if arrays or subroutines exist.  Use
          on predefined variables is not guaranteed to produce intuitive
          results.    Examples:

           print if defined $switch{'D'};
           print "$val\n" while defined($val = pop(@ary));
           die "Can't readlink $sym: $!"
            unless defined($value = readlink $sym);
           eval '@foo = ()' if defined(@foo);
           die "No XYZ package defined" unless defined %_XYZ;
           sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }

          See also undef.

      delete $ASSOC{KEY}
          Deletes the specified value from the specified associative
          array.  Returns the deleted value, or the undefined value if
          nothing was deleted.  Deleting from $ENV{} modifies the
          environment.  Deleting from an array bound to a dbm file
          deletes the entry from the dbm file.

          The following deletes all the values of an associative array:

           foreach $key (keys %ARRAY) {
            delete $ARRAY{$key};
           }

          (But it would be faster to use the reset command.  Saying
          undef %ARRAY is faster yet.)

      die(LIST)

      die LIST
          Outside of an eval, prints the value of LIST to STDERR and
          exits with the current value of $!  (errno).  If $! is 0,
          exits with the value of ($? >> 8) (`command` status).  If ($?
          >> 8) is 0, exits with 255.  Inside an eval, the error message
          is stuffed into $@ and the eval is terminated with the
          undefined value.

          Equivalent examples:

           die "Can't cd to spool: $!\n"
            unless chdir '/usr/spool/news';

           chdir '/usr/spool/news' || die "Can't cd to spool: $!\n"


          If the value of EXPR does not end in a newline, the current
          script line number and input line number (if any) are also
          printed, and a newline is supplied.  Hint: sometimes appending
          ", stopped" to your message will cause it to make better sense
          when the string "at foo line 123" is appended.  Suppose you
          are running script "canasta".

           die "/etc/games is no good";
           die "/etc/games is no good, stopped";

          produce, respectively

           /etc/games is no good at canasta line 123.
           /etc/games is no good, stopped at canasta line 123.

          See also exit.

      do BLOCK
          Returns the value of the last command in the sequence of
          commands indicated by BLOCK.  When modified by a loop
          modifier, executes the BLOCK once before testing the loop
          condition.  (On other statements the loop modifiers test the
          conditional first.)

      do SUBROUTINE (LIST)
          Executes a SUBROUTINE declared by a sub declaration, and
          returns the value of the last expression evaluated in
          SUBROUTINE.  If there is no subroutine by that name, produces
          a fatal error.  (You may use the "defined" operator to
          determine if a subroutine exists.) If you pass arrays as part
          of LIST you may wish to pass the length of the array in front
          of each array.  (See the section on subroutines later on.) The
          parentheses are required to avoid confusion with the "do EXPR"
          form.

          SUBROUTINE may also be a single scalar variable, in which case
          the name of the subroutine to execute is taken from the
          variable.

          As an alternate (and preferred) form, you may call a
          subroutine by prefixing the name with an ampersand:
          &foo(@args).  If you aren't passing any arguments, you don't
          have to use parentheses.    If you omit the parentheses, no @_
          array is passed to the subroutine.  The & form is also used to
          specify subroutines to the defined and undef operators:

           if (defined &$var) { &$var($parm); undef &$var; }


      do EXPR Uses the value of EXPR as a filename and executes the contents
          of the file as a perl script.  Its primary use is to include
          subroutines from a perl subroutine library.

           do 'stat.pl';

          is just like

           eval `cat stat.pl`;

          except that it's more efficient, more concise, keeps track of
          the current filename for error messages, and searches all the
          -I libraries if the file isn't in the current directory (see
          also the @INC array in Predefined Names).  It's the same,
          however, in that it does reparse the file every time you call
          it, so if you are going to use the file inside a loop you
          might prefer to use -P and #include, at the expense of a
          little more startup time.  (The main problem with #include is
          that cpp doesn't grok # comments--a workaround is to use ";#"
          for standalone comments.) Note that the following are NOT
          equivalent:

           do $foo;  # eval a file
           do $foo();      # call a subroutine

          Note that inclusion of library routines is better done with
          the "require" operator.

      dump LABEL
          This causes an immediate core dump.  Primarily this is so that
          you can use the undump program to turn your core dump into an
          executable binary after having initialized all your variables
          at the beginning of the program.    When the new binary is
          executed it will begin by executing a "goto LABEL" (with all
          the restrictions that goto suffers).  Think of it as a goto
          with an intervening core dump and reincarnation.    If LABEL is
          omitted, restarts the program from the top.  WARNING: any
          files opened at the time of the dump will NOT be open any more
          when the program is reincarnated, with possible resulting
          confusion on the part of perl.  See also -u.

          Example:

           #!/usr/bin/perl
           require 'getopt.pl';
           require 'stat.pl';
           %days = (
               'Sun',1,
               'Mon',2,
               'Tue',3,
               'Wed',4,
               'Thu',5,
               'Fri',6,
               'Sat',7);

           dump QUICKSTART if $ARGV[0] eq '-d';

          QUICKSTART:
           do Getopt('f');


      each(ASSOC_ARRAY)

      each ASSOC_ARRAY
          Returns a 2 element array consisting of the key and value for
          the next value of an associative array, so that you can
          iterate over it.    Entries are returned in an apparently random
          order.  When the array is entirely read, a null array is
          returned (which when assigned produces a FALSE (0) value).
          The next call to each() after that will start iterating again.
          The iterator can be reset only by reading all the elements
          from the array.  You must not modify the array while iterating
          over it.    There is a single iterator for each associative
          array, shared by all each(), keys() and values() function
          calls in the program.  The following prints out your
          environment like the printenv program, only in a different
          order:

           while (($key,$value) = each %ENV) {
            print "$key=$value\n";
           }

          See also keys() and values().

      eof(FILEHANDLE)

      eof()

      eof     Returns 1 if the next read on FILEHANDLE will return end of
          file, or if FILEHANDLE is not open.  FILEHANDLE may be an
          expression whose value gives the real filehandle name.  (Note
          that this function actually reads a character and then
          ungetc's it, so it is not very useful in an interactive
          context.) An eof without an argument returns the eof status
          for the last file read.  Empty parentheses () may be used to
          indicate the pseudo file formed of the files listed on the
          command line, i.e. eof() is reasonable to use inside a while
          (<>) loop to detect the end of only the last file.  Use
          eof(ARGV) or eof without the parentheses to test EACH file in
          a while (<>) loop.  Examples:

           # insert dashes just before last line of last file
           while (<>) {
            if (eof()) {
                 print "--------------\n";
            }
            print;
           }

           # reset line numbering on each input file
           while (<>) {
            print "$.\t$_";
            if (eof) {     # Not eof().
                 close(ARGV);
            }
           }


      eval(EXPR)

      eval EXPR

      eval BLOCK
          EXPR is parsed and executed as if it were a little perl
          program.    It is executed in the context of the current perl
          program, so that any variable settings, subroutine or format
          definitions remain afterwards.  The value returned is the
          value of the last expression evaluated, just as with
          subroutines.  If there is a syntax error or runtime error, or
          a die statement is executed, an undefined value is returned by
          eval, and $@ is set to the error message.  If there was no
          error, $@ is guaranteed to be a null string.  If EXPR is
          omitted, evaluates $_.  The final semicolon, if any, may be
          omitted from the expression.

          Note that, since eval traps otherwise-fatal errors, it is
          useful for determining whether a particular feature (such as
          dbmopen or symlink) is implemented.  It is also Perl's
          exception trapping mechanism, where the die operator is used
          to raise exceptions.

          If the code to be executed doesn't vary, you may use the
          eval-BLOCK form to trap run-time errors without incurring the
          penalty of recompiling each time.  The error, if any, is still
          returned in $@.  Evaluating a single-quoted string (as EXPR)
          has the same effect, except that the eval-EXPR form reports
          syntax errors at run time via $@, whereas the eval-BLOCK form
          reports syntax errors at compile time.  The eval-EXPR form is
          optimized to eval-BLOCK the first time it succeeds.  (Since
          the replacement side of a substitution is considered a
          single-quoted string when you use the e modifier, the same
          optimization occurs there.)  Examples:

           # make divide-by-zero non-fatal
           eval { $answer = $a / $b; }; warn $@ if $@;

           # optimized to same thing after first use
           eval '$answer = $a / $b'; warn $@ if $@;

           # a compile-time error
           eval { $answer = };

           # a run-time error
           eval '$answer =';   # sets $@


      exec(LIST)

      exec LIST
          If there is more than one argument in LIST, or if LIST is an
          array with more than one value, calls execvp() with the
          arguments in LIST.  If there is only one scalar argument, the
          argument is checked for shell metacharacters.  If there are
          any, the entire argument is passed to "/bin/sh -c" for
          parsing.    If there are none, the argument is split into words
          and passed directly to execvp(), which is more efficient.
          Note: exec (and system) do not flush your output buffer, so
          you may need to set $| to avoid lost output.  Examples:

           exec '/bin/echo', 'Your arguments are: ', @ARGV;
           exec "sort $outfile | uniq";


          If you don't really want to execute the first argument, but
          want to lie to the program you are executing about its own
          name, you can specify the program you actually want to run by
          assigning that to a variable and putting the name of the
          variable in front of the LIST without a comma.  (This always
          forces interpretation of the LIST as a multi-valued list, even
          if there is only a single scalar in the list.) Example:

           $shell = '/bin/csh';
           exec $shell '-sh';        # pretend it's a login shell

      exit(EXPR)

      exit EXPR
          Evaluates EXPR and exits immediately with that value.
          Example:

           $ans = <STDIN>;
           exit 0 if $ans =~ /^[Xx]/;

          See also die.  If EXPR is omitted, exits with 0 status.

      exp(EXPR)

      exp EXPR
          Returns e to the power of EXPR.  If EXPR is omitted, gives
          exp($_).

      fcntl(FILEHANDLE,FUNCTION,SCALAR)
          Implements the fcntl(2) function.  You'll probably have to say

           require "fcntl.ph"; # probably /usr/local/lib/perl/fcntl.ph

          first to get the correct function definitions.  If fcntl.ph
          doesn't exist or doesn't have the correct definitions you'll
          have to roll your own, based on your C header files such as
          <sys/fcntl.h>.  (There is a perl script called h2ph that comes
          with the perl kit which may help you in this.) Argument
          processing and value return works just like ioctl below.    Note
          that fcntl will produce a fatal error if used on a machine
          that doesn't implement fcntl(2).

      fileno(FILEHANDLE)

      fileno FILEHANDLE
          Returns the file descriptor for a filehandle.  Useful for
          constructing bitmaps for select().  If FILEHANDLE is an
          expression, the value is taken as the name of the filehandle.

      flock(FILEHANDLE,OPERATION)
          Calls flock(2) on FILEHANDLE.  See manual page for flock(2)
          for definition of OPERATION.  Returns true for success, false
          on failure.  Will produce a fatal error if used on a machine
          that doesn't implement flock(2).    Here's a mailbox appender
          for BSD systems.

           $LOCK_SH = 1;
           $LOCK_EX = 2;
           $LOCK_NB = 4;
           $LOCK_UN = 8;

           sub lock {
               flock(MBOX,$LOCK_EX);
               # and, in case someone appended
               # while we were waiting...
               seek(MBOX, 0, 2);
           }

           sub unlock {
               flock(MBOX,$LOCK_UN);
           }

           open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
            || die "Can't open mailbox: $!";

           do lock();
           print MBOX $msg,"\n\n";
           do unlock();


      fork    Does a fork() call.  Returns the child pid to the parent
          process and 0 to the child process.  Note: unflushed buffers
          remain unflushed in both processes, which means you may need
          to set $| to avoid duplicate output.

      getc(FILEHANDLE)

      getc FILEHANDLE

      getc    Returns the next character from the input file attached to
          FILEHANDLE, or a null string at EOF.  If FILEHANDLE is
          omitted, reads from STDIN.

      getlogin
          Returns the current login from /etc/utmp, if any.  If null,
          use getpwuid.

           $login = getlogin || (getpwuid($<))[0] || "Somebody";


      getpeername(SOCKET)
          Returns the packed sockaddr address of other end of the SOCKET
          connection.

           # An internet sockaddr
           $sockaddr = 'S n a4 x8';
           $hersockaddr = getpeername(S);
           ($family, $port, $heraddr) =
                 unpack($sockaddr,$hersockaddr);


      getpgrp(PID)

      getpgrp PID
          Returns the current process group for the specified PID, 0 for
          the current process.  Will produce a fatal error if used on a
          machine that doesn't implement getpgrp(2).  If EXPR is
          omitted, returns process group of current process.

      getppid Returns the process id of the parent process.

      getpriority(WHICH,WHO)
          Returns the current priority for a process, a process group,
          or a user.  (See getpriority(2).) Will produce a fatal error
          if used on a machine that doesn't implement getpriority(2).

      getpwnam(NAME)

      getgrnam(NAME)

      gethostbyname(NAME)

      getnetbyname(NAME)

      getprotobyname(NAME)

      getpwuid(UID)

      getgrgid(GID)

      getservbyname(NAME,PROTO)

      gethostbyaddr(ADDR,ADDRTYPE)

      getnetbyaddr(ADDR,ADDRTYPE)

      getprotobynumber(NUMBER)

      getservbyport(PORT,PROTO)

      getpwent

      getgrent

      gethostent

      getnetent

      getprotoent

      getservent

      setpwent

      setgrent

      sethostent(STAYOPEN)

      setnetent(STAYOPEN)

      setprotoent(STAYOPEN)

      setservent(STAYOPEN)

      endpwent

      endgrent

      endhostent

      endnetent

      endprotoent

      endservent
          These routines perform the same functions as their
          counterparts in the system library.  Within an array context,
          the return values from the various get routines are as
          follows:

           ($name,$passwd,$uid,$gid,
              $quota,$comment,$gcos,$dir,$shell) = getpw...
           ($name,$passwd,$gid,$members) = getgr...
           ($name,$aliases,$addrtype,$length,@addrs) = gethost...
           ($name,$aliases,$addrtype,$net) = getnet...
           ($name,$aliases,$proto) = getproto...
           ($name,$aliases,$port,$proto) = getserv...

          (If the entry doesn't exist you get a null list.)

          Within a scalar context, you get the name, unless the function
          was a lookup by name, in which case you get the other thing,
          whatever it is.  (If the entry doesn't exist you get the
          undefined value.) For example:

           $uid = getpwnam
           $name = getpwuid
           $name = getpwent
           $gid = getgrnam
           $name = getgrgid
           $name = getgrent
           etc.

          The $members value returned by getgr... is a space separated
          list of the login names of the members of the group.

          For the gethost... functions, if the h_errno variable is
          supported in C, it will be returned to you via $? if the
          function call fails.  The @addrs value returned by a
          successful call is a list of the raw addresses returned by the
          corresponding system library call.  In the Internet domain,
          each address is four bytes long and you can unpack it by
          saying something like:

           ($a,$b,$c,$d) = unpack('C4',$addr[0]);


      getsockname(SOCKET)
          Returns the packed sockaddr address of this end of the SOCKET
          connection.

           # An internet sockaddr
           $sockaddr = 'S n a4 x8';
           $mysockaddr = getsockname(S);
           ($family, $port, $myaddr) =
                 unpack($sockaddr,$mysockaddr);


      getsockopt(SOCKET,LEVEL,OPTNAME)
          Returns the socket option requested, or undefined if there is
          an error.

      gmtime(EXPR)

      gmtime EXPR
          Converts a time as returned by the time function to a 9-
          element array with the time analyzed for the Greenwich
          timezone.  Typically used as follows:

          ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                        gmtime(time);

          All array elements are numeric, and come straight out of a
          struct tm.  In particular this means that $mon has the range
          0..11 and $wday has the range 0..6.  If EXPR is omitted, does
          gmtime(time).

      goto LABEL
          Finds the statement labeled with LABEL and resumes execution
          there.  Currently you may only go to statements in the main
          body of the program that are not nested inside a do {}
          construct.  This statement is not implemented very
          efficiently, and is here only to make the sed-to-perl
          translator easier.  I may change its semantics at any time,
          consistent with support for translated sed scripts.  Use it at
          your own risk.  Better yet, don't use it at all.

      grep(EXPR,LIST)
          Evaluates EXPR for each element of LIST (locally setting $_ to
          each element) and returns the array value consisting of those
          elements for which the expression evaluated to true.  In a
          scalar context, returns the number of times the expression was
          true.

           @foo = grep(!/^#/, @bar);    # weed out comments

          Note that, since $_ is a reference into the array value, it
          can be used to modify the elements of the array.    While this
          is useful and supported, it can cause bizarre results if the
          LIST is not a named array.

      hex(EXPR)

      hex EXPR
          Returns the decimal value of EXPR interpreted as an hex
          string.  (To interpret strings that might start with 0 or 0x
          see oct().) If EXPR is omitted, uses $_.

      index(STR,SUBSTR,POSITION)

      index(STR,SUBSTR)
          Returns the position of the first occurrence of SUBSTR in STR
          at or after POSITION.  If POSITION is omitted, starts
          searching from the beginning of the string.  The return value
          is based at 0, or whatever you've set the $[ variable to.  If
          the substring is not found, returns one less than the base,
          ordinarily -1.

      int(EXPR)

      int EXPR
          Returns the integer portion of EXPR.  If EXPR is omitted, uses
          $_.

      ioctl(FILEHANDLE,FUNCTION,SCALAR)
          Implements the ioctl(2) function.  You'll probably have to say

           require "ioctl.ph"; # probably /usr/local/lib/perl/ioctl.ph

          first to get the correct function definitions.  If ioctl.ph
          doesn't exist or doesn't have the correct definitions you'll
          have to roll your own, based on your C header files such as
          <sys/ioctl.h>.  (There is a perl script called h2ph that comes
          with the perl kit which may help you in this.) SCALAR will be
          read and/or written depending on the FUNCTION--a pointer to
          the string value of SCALAR will be passed as the third
          argument of the actual ioctl call.  (If SCALAR has no string
          value but does have a numeric value, that value will be passed
          rather than a pointer to the string value.  To guarantee this
          to be true, add a 0 to the scalar before using it.) The pack()
          and unpack() functions are useful for manipulating the values
          of structures used by ioctl().  The following example sets the
          erase character to DEL.

           require 'ioctl.ph';
           $sgttyb_t = "ccccs";      # 4 chars and a short
           if (ioctl(STDIN,$TIOCGETP,$sgttyb)) {
            @ary = unpack($sgttyb_t,$sgttyb);
            $ary[2] = 127;
            $sgttyb = pack($sgttyb_t,@ary);
            ioctl(STDIN,$TIOCSETP,$sgttyb)
                 || die "Can't ioctl: $!";
           }

          The return value of ioctl (and fcntl) is as follows:

           if OS returns:        perl returns:
             -1               undefined value
             0                  string "0 but true"
             anything else          that number

          Thus perl returns true on success and false on failure, yet
          you can still easily determine the actual value returned by
          the operating system:

           ($retval = ioctl(...)) || ($retval = -1);
           printf "System returned %d\n", $retval;

      join(EXPR,LIST)

      join(EXPR,ARRAY)
          Joins the separate strings of LIST or ARRAY into a single
          string with fields separated by the value of EXPR, and returns
          the string.  Example:

          $_ = join(':',
            $login,$passwd,$uid,$gid,$gcos,$home,$shell);

          See split.

      keys(ASSOC_ARRAY)

      keys ASSOC_ARRAY
          Returns a normal array consisting of all the keys of the named
          associative array.  The keys are returned in an apparently
          random order, but it is the same order as either the values()
          or each() function produces (given that the associative array
          has not been modified).  Here is yet another way to print your
          environment:

           @keys = keys %ENV;
           @values = values %ENV;
           while ($#keys >= 0) {
            print pop(@keys), '=', pop(@values), "\n";
           }

          or how about sorted by key:

           foreach $key (sort(keys %ENV)) {
            print $key, '=', $ENV{$key}, "\n";
           }


      kill(LIST)

      kill LIST
          Sends a signal to a list of processes.  The first element of
          the list must be the signal to send.  Returns the number of
          processes successfully signaled.

           $cnt = kill 1, $child1, $child2;
           kill 9, @goners;

          If the signal is negative, kills process groups instead of
          processes.  (On System V, a negative process number will also
          kill process groups, but that's not portable.) You may use a
          signal name in quotes.

      last LABEL

      last    The last command is like the break statement in C (as used in
          loops); it immediately exits the loop in question.  If the
          LABEL is omitted, the command refers to the innermost
          enclosing loop.  The continue block, if any, is not executed:

           line: while (<STDIN>) {
            last line if /^$/;  # exit when done with header
            ...
           }

      length(EXPR)

      length EXPR
          Returns the length in characters of the value of EXPR.  If
          EXPR is omitted, returns length of $_.

      link(OLDFILE,NEWFILE)
          Creates a new filename linked to the old filename.  Returns 1
          for success, 0 otherwise.

      listen(SOCKET,QUEUESIZE)
          Does the same thing that the listen system call does.  Returns
          true if it succeeded, false otherwise.  See example in section
          on Interprocess Communication.

      local(LIST)
          Declares the listed variables to be local to the enclosing
          block, subroutine, eval or "do".    All the listed elements must
          be legal lvalues.  This operator works by saving the current
          values of those variables in LIST on a hidden stack and
          restoring them upon exiting the block, subroutine or eval.
          This means that called subroutines can also reference the
          local variable, but not the global one.  The LIST may be
          assigned to if desired, which allows you to initialize your
          local variables.    (If no initializer is given for a particular
          variable, it is created with an undefined value.) Commonly
          this is used to name the parameters to a subroutine.
          Examples:

           sub RANGEVAL {
            local($min, $max, $thunk) = @_;
            local($result) = '';
            local($i);

            # Presumably $thunk makes reference to $i

            for ($i = $min; $i < $max; $i++) {
                 $result .= eval $thunk;
            }

            $result;
           }

           if ($sw eq '-v') {
               # init local array with global array
               local(@ARGV) = @ARGV;
               unshift(@ARGV,'echo');
               system @ARGV;
           }
           # @ARGV restored

           # temporarily add to digits associative array
           if ($base12) {
            # (NOTE: not claiming this is efficient!)
            local(%digits) = (%digits,'t',10,'e',11);
            do parse_num();
           }

          Note that local() is a run-time command, and so gets executed
          every time through a loop, using up more stack storage each
          time until it's all released at once when the loop is exited.

      localtime(EXPR)

      localtime EXPR
          Converts a time as returned by the time function to a 9-
          element array with the time analyzed for the local timezone.
          Typically used as follows:

          ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                        localtime(time);

          All array elements are numeric, and come straight out of a
          struct tm.  In particular this means that $mon has the range
          0..11 and $wday has the range 0..6.  If EXPR is omitted, does
          localtime(time).

      log(EXPR)

      log EXPR
          Returns logarithm (base e) of EXPR.  If EXPR is omitted,
          returns log of $_.

      lstat(FILEHANDLE)

      lstat FILEHANDLE

      lstat(EXPR)

      lstat SCALARVARIABLE
          Does the same thing as the stat() function, but stats a
          symbolic link instead of the file the symbolic link points to.
          If symbolic links are unimplemented on your system, a normal
          stat is done.

      m/PATTERN/gio

      /PATTERN/gio
          Searches a string for a pattern match, and returns true (1) or
          false ('').  If no string is specified via the =~ or !~
          operator, the $_ string is searched.  (The string specified
          with =~ need not be an lvalue--it may be the result of an
          expression evaluation, but remember the =~ binds rather
          tightly.) See also the section on regular expressions.

          If / is the delimiter then the initial 'm' is optional.  With
          the 'm' you can use any pair of non-alphanumeric characters as
          delimiters.  This is particularly useful for matching Unix
          path names that contain '/'.  If the final delimiter is
          followed by the optional letter 'i', the matching is done in a
          case-insensitive manner.    PATTERN may contain references to
          scalar variables, which will be interpolated (and the pattern
          recompiled) every time the pattern search is evaluated.  (Note
          that $) and $| may not be interpolated because they look like
          end-of-string tests.) If you want such a pattern to be
          compiled only once, add an "o" after the trailing delimiter.
          This avoids expensive run-time recompilations, and is useful
          when the value you are interpolating won't change over the
          life of the script.  If the PATTERN evaluates to a null
          string, the most recent successful regular expression is used
          instead.

          If used in a context that requires an array value, a pattern
          match returns an array consisting of the subexpressions
          matched by the parentheses in the pattern, i.e. ($1, $2,
          $3...).  It does NOT actually set $1, $2, etc. in this case,
          nor does it set $+, $`, $& or $'.  If the match fails, a null
          array is returned.  If the match succeeds, but there were no
          parentheses, an array value of (1) is returned.

          Examples:

          open(tty, '/dev/tty');
          <tty> =~ /^y/i && do foo();     # do foo if desired

          if (/Version: *([0-9.]*)/) { $version = $1; }

          next if m#^/usr/spool/uucp#;

          # poor man's grep
          $arg = shift;
          while (<>) {
               print if /$arg/o;    # compile only once
          }

          if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))

          This last example splits $foo into the first two words and the
          remainder of the line, and assigns those three fields to $F1,
          $F2 and $Etc.  The conditional is true if any variables were
          assigned, i.e. if the pattern matched.

          The "g" modifier specifies global pattern matching--that is,
          matching as many times as possible within the string.  How it
          behaves depends on the context.  In an array context, it
          returns a list of all the substrings matched by all the
          parentheses in the regular expression.  If there are no
          parentheses, it returns a list of all the matched strings, as
          if there were parentheses around the whole pattern.  In a
          scalar context, it iterates through the string, returning TRUE
          each time it matches, and FALSE when it eventually runs out of
          matches.    (In other words, it remembers where it left off last
          time and restarts the search at that point.)  It presumes that
          you have not modified the string since the last match.
          Modifying the string between matches may result in undefined
          behavior.  (You can actually get away with in-place
          modifications via substr() that do not change the length of
          the entire string.  In general, however, you should be using
          s///g for such modifications.)  Examples:

           # array context
           ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);

           # scalar context
           $/ = ""; $* = 1;
           while ($paragraph = <>) {
               while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
            $sentences++;
               }
           }
           print "$sentences\n";


      mkdir(FILENAME,MODE)
          Creates the directory specified by FILENAME, with permissions
          specified by MODE (as modified by umask).  If it succeeds it
          returns 1, otherwise it returns 0 and sets $! (errno).

      msgctl(ID,CMD,ARG)
          Calls the System V IPC function msgctl.  If CMD is &IPC_STAT,
          then ARG must be a variable which will hold the returned
          msqid_ds structure.  Returns like ioctl: the undefined value
          for error, "0 but true" for zero, or the actual return value
          otherwise.

      msgget(KEY,FLAGS)
          Calls the System V IPC function msgget.  Returns the message
          queue id, or the undefined value if there is an error.

      msgsnd(ID,MSG,FLAGS)
          Calls the System V IPC function msgsnd to send the message MSG
          to the message queue ID.    MSG must begin with the long integer
          message type, which may be created with pack("L", $type).
          Returns true if successful, or false if there is an error.

      msgrcv(ID,VAR,SIZE,TYPE,FLAGS)
          Calls the System V IPC function msgrcv to receive a message
          from message queue ID into variable VAR with a maximum message
          size of SIZE.  Note that if a message is received, the message
          type will be the first thing in VAR, and the maximum length of
          VAR is SIZE plus the size of the message type.  Returns true
          if successful, or false if there is an error.

      next LABEL

      next    The next command is like the continue statement in C; it
          starts the next iteration of the loop:

           line: while (<STDIN>) {
            next line if /^#/;  # discard comments
            ...
           }

          Note that if there were a continue block on the above, it
          would get executed even on discarded lines.  If the LABEL is
          omitted, the command refers to the innermost enclosing loop.

      oct(EXPR)

      oct EXPR
          Returns the decimal value of EXPR interpreted as an octal
          string.  (If EXPR happens to start off with 0x, interprets it
          as a hex string instead.) The following will handle decimal,
          octal and hex in the standard notation:

           $val = oct($val) if $val =~ /^0/;

          If EXPR is omitted, uses $_.

      open(FILEHANDLE,EXPR)

      open(FILEHANDLE)

      open FILEHANDLE
          Opens the file whose filename is given by EXPR, and associates
          it with FILEHANDLE.  If FILEHANDLE is an expression, its value
          is used as the name of the real filehandle wanted.  If EXPR is
          omitted, the scalar variable of the same name as the
          FILEHANDLE contains the filename.  If the filename begins with
          "<" or nothing, the file is opened for input.  If the filename
          begins with ">", the file is opened for output.  If the
          filename begins with ">>", the file is opened for appending.
          (You can put a '+' in front of the '>' or '<' to indicate that
          you want both read and write access to the file.) If the
          filename begins with "|", the filename is interpreted as a
          command to which output is to be piped, and if the filename
          ends with a "|", the filename is interpreted as command which
          pipes input to us.  (You may not have a command that pipes
          both in and out.) Opening '-' opens STDIN and opening '>-'
          opens STDOUT.  Open returns non-zero upon success, the
          undefined value otherwise.  If the open involved a pipe, the
          return value happens to be the pid of the subprocess.
          Examples:

           $article = 100;
           open article || die "Can't find article $article: $!\n";
           while (<article>) {...

           open(LOG, '>>/usr/spool/news/twitlog');
                       # (log is reserved)

           open(article, "caesar <$article |");
                       # decrypt article

           open(extract, "|sort >/tmp/Tmp$$");
                       # $$ is our process#

           # process argument list of files along with any includes

           foreach $file (@ARGV) {
            do process($file, 'fh00');    # no pun intended
           }

           sub process {
            local($filename, $input) = @_;
            $input++;      # this is a string increment
            unless (open($input, $filename)) {
                 print STDERR "Can't open $filename: $!\n";
                 return;
            }
            while (<$input>) {     # note use of indirection
                 if (/^#include "(.*)"/) {
                  do process($1, $input);
                  next;
                 }
                 ...       # whatever
            }
           }

          You may also, in the Bourne shell tradition, specify an EXPR
          beginning with ">&", in which case the rest of the string is
          interpreted as the name of a filehandle (or file descriptor,
          if numeric) which is to be duped and opened.  You may use &
          after >, >>, <, +>, +>> and +<.  The mode you specify should
          match the mode of the original filehandle.  Here is a script
          that saves, redirects, and restores STDOUT and STDERR:

           #!/usr/bin/perl
           open(SAVEOUT, ">&STDOUT");
           open(SAVEERR, ">&STDERR");

           open(STDOUT, ">foo.out") || die "Can't redirect stdout";
           open(STDERR, ">&STDOUT") || die "Can't dup stdout";

           select(STDERR); $| = 1;     # make unbuffered
           select(STDOUT); $| = 1;     # make unbuffered

           print STDOUT "stdout 1\n";     # this works for
           print STDERR "stderr 1\n";     # subprocesses too

           close(STDOUT);
           close(STDERR);

           open(STDOUT, ">&SAVEOUT");
           open(STDERR, ">&SAVEERR");

           print STDOUT "stdout 2\n";
           print STDERR "stderr 2\n";

          If you open a pipe on the command "-", i.e. either "|-" or
          "-|", then there is an implicit fork done, and the return
          value of open is the pid of the child within the parent
          process, and 0 within the child process.    (Use defined($pid)
          to determine if the open was successful.) The filehandle
          behaves normally for the parent, but i/o to that filehandle is
          piped from/to the STDOUT/STDIN of the child process.  In the
          child process the filehandle isn't opened--i/o happens from/to
          the new STDOUT or STDIN.    Typically this is used like the
          normal piped open when you want to exercise more control over
          just how the pipe command gets executed, such as when you are
          running setuid, and don't want to have to scan shell commands
          for metacharacters.  The following pairs are more or less
          equivalent:

           open(FOO, "|tr '[a-z]' '[A-Z]'");
           open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';

           open(FOO, "cat -n '$file'|");
           open(FOO, "-|") || exec 'cat', '-n', $file;

          Explicitly closing any piped filehandle causes the parent
          process to wait for the child to finish, and returns the
          status value in $?.  Note: on any operation which may do a
          fork, unflushed buffers remain unflushed in both processes,
          which means you may need to set $| to avoid duplicate output.

          The filename that is passed to open will have leading and
          trailing whitespace deleted.  In order to open a file with
          arbitrary weird characters in it, it's necessary to protect
          any leading and trailing whitespace thusly:

              $file =~ s#^(\s)#./$1#;
              open(FOO, "< $file\0");


      opendir(DIRHANDLE,EXPR)
          Opens a directory named EXPR for processing by readdir(),
          telldir(), seekdir(), rewinddir() and closedir().  Returns
          true if successful.  DIRHANDLEs have their own namespace
          separate from FILEHANDLEs.

      ord(EXPR)

      ord EXPR
          Returns the numeric ascii value of the first character of
          EXPR.  If EXPR is omitted, uses $_.

      pack(TEMPLATE,LIST)
          Takes an array or list of values and packs it into a binary
          structure, returning the string containing the structure.  The
          TEMPLATE is a sequence of characters that give the order and
          type of values, as follows:

           A    An ascii string, will be space padded.
           a    An ascii string, will be null padded.
           c    A signed char value.
           C    An unsigned char value.
           s    A signed short value.
           S    An unsigned short value.
           i    A signed integer value.
           I    An unsigned integer value.
           l    A signed long value.
           L    An unsigned long value.
           n    A short in "network" order.
           N    A long in "network" order.
           f    A single-precision float in the native format.
           d    A double-precision float in the native format.
           p    A pointer to a string.
           v    A short in "VAX" (little-endian) order.
           V    A long in "VAX" (little-endian) order.
           x    A null byte.
           X    Back up a byte.
           @    Null fill to absolute position.
           u    A uuencoded string.
           b    A bit string (ascending bit order, like vec()).
           B    A bit string (descending bit order).
           h    A hex string (low nybble first).
           H    A hex string (high nybble first).

          Each letter may optionally be followed by a number which gives
          a repeat count.  With all types except "a", "A", "b", "B", "h"
          and "H", the pack function will gobble up that many values
          from the LIST.  A * for the repeat count means to use however
          many items are left.  The "a" and "A" types gobble just one
          value, but pack it as a string of length count, padding with
          nulls or spaces as necessary.  (When unpacking, "A" strips
          trailing spaces and nulls, but "a" does not.) Likewise, the
          "b" and "B" fields pack a string that many bits long.  The "h"
          and "H" fields pack a string that many nybbles long.  Real
          numbers (floats and doubles) are in the native machine format
          only; due to the multiplicity of floating formats around, and
          the lack of a standard "network" representation, no facility
          for interchange has been made.  This means that packed
          floating point data written on one machine may not be readable
          on another - even if both use IEEE floating point arithmetic
          (as the endian-ness of the memory representation is not part
          of the IEEE spec).  Note that perl uses doubles internally for
          all numeric calculation, and converting from double -> float
          -> double will lose precision (i.e. unpack("f", pack("f",
          $foo)) will not in general equal $foo).
          Examples:

           $foo = pack("cccc",65,66,67,68);
           # foo eq "ABCD"
           $foo = pack("c4",65,66,67,68);
           # same thing

           $foo = pack("ccxxcc",65,66,67,68);
           # foo eq "AB\0\0CD"

           $foo = pack("s2",1,2);
           # "\1\0\2\0" on little-endian
           # "\0\1\0\2" on big-endian

           $foo = pack("a4","abcd","x","y","z");
           # "abcd"

           $foo = pack("aaaa","abcd","x","y","z");
           # "axyz"

           $foo = pack("a14","abcdefg");
           # "abcdefg\0\0\0\0\0\0\0"

           $foo = pack("i9pl", gmtime);
           # a real struct tm (on my system anyway)

           sub bintodec {
               unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
           }
          The same template may generally also be used in the unpack
          function.

      pipe(READHANDLE,WRITEHANDLE)
          Opens a pair of connected pipes like the corresponding system
          call.  Note that if you set up a loop of piped processes,
          deadlock can occur unless you are very careful.  In addition,
          note that perl's pipes use stdio buffering, so you may need to
          set $| to flush your WRITEHANDLE after each command, depending
          on the application.  [Requires version 3.0 patchlevel 9.]

      pop(ARRAY)

      pop ARRAY
          Pops and returns the last value of the array, shortening the
          array by 1.  Has the same effect as

           $tmp = $ARRAY[$#ARRAY--];

          If there are no elements in the array, returns the undefined
          value.

      print(FILEHANDLE LIST)

      print(LIST)

      print FILEHANDLE LIST

      print LIST

      print   Prints a string or a comma-separated list of strings.  Returns
          non-zero if successful.  FILEHANDLE may be a scalar variable
          name, in which case the variable contains the name of the
          filehandle, thus introducing one level of indirection.  (NOTE:
          If FILEHANDLE is a variable and the next token is a term, it
          may be misinterpreted as an operator unless you interpose a +
          or put parens around the arguments.) If FILEHANDLE is omitted,
          prints by default to standard output (or to the last selected
          output channel--see select()).  If LIST is also omitted,
          prints $_ to STDOUT.  To set the default output channel to
          something other than STDOUT use the select operation.  Note
          that, because print takes a LIST, anything in the LIST is
          evaluated in an array context, and any subroutine that you
          call will have one or more of its expressions evaluated in an
          array context.  Also be careful not to follow the print
          keyword with a left parenthesis unless you want the
          corresponding right parenthesis to terminate the arguments to
          the print--interpose a + or put parens around all the
          arguments.

      printf(FILEHANDLE LIST)

      printf(LIST)

      printf FILEHANDLE LIST

      printf LIST
          Equivalent to a "print FILEHANDLE sprintf(LIST)".

      push(ARRAY,LIST)
          Treats ARRAY (@ is optional) as a stack, and pushes the values
          of LIST onto the end of ARRAY.  The length of ARRAY increases
          by the length of LIST.  Has the same effect as

          for $value (LIST) {
               $ARRAY[++$#ARRAY] = $value;
          }

          but is more efficient.

      q/STRING/

      qq/STRING/

      qx/STRING/
          These are not really functions, but simply syntactic sugar to
          let you avoid putting too many backslashes into quoted
          strings.    The q operator is a generalized single quote, and
          the qq operator a generalized double quote.  The qx operator
          is a generalized backquote.  Any non-alphanumeric delimiter
          can be used in place of /, including newline.  If the
          delimiter is an opening bracket or parenthesis, the final
          delimiter will be the corresponding closing bracket or
          parenthesis.  (Embedded occurrences of the closing bracket
          need to be backslashed as usual.) Examples:

           $foo = q!I said, "You said, 'She said it.'"!;
           $bar = q('This is it.');
           $today = qx{ date };
           $_ .= qq
          *** The previous line contains the naughty word "$&".\n
            if /(ibm|apple|awk)/;       # :-)

      rand(EXPR)

      rand EXPR

      rand    Returns a random fractional number between 0 and the value of
          EXPR.  (EXPR should be positive.) If EXPR is omitted, returns
          a value between 0 and 1.    See also srand().

      read(FILEHANDLE,SCALAR,LENGTH,OFFSET)

      read(FILEHANDLE,SCALAR,LENGTH)
          Attempts to read LENGTH bytes of data into variable SCALAR
          from the specified FILEHANDLE.  Returns the number of bytes
          actually read, or undef if there was an error.  SCALAR will be
          grown or shrunk to the length actually read.  An OFFSET may be
          specified to place the read data at some other place than the
          beginning of the string.    This call is actually implemented in
          terms of stdio's fread call.  To get a true read system call,
          see sysread.

      readdir(DIRHANDLE)

      readdir DIRHANDLE
          Returns the next directory entry for a directory opened by
          opendir().  If used in an array context, returns all the rest
          of the entries in the directory.    If there are no more
          entries, returns an undefined value in a scalar context or a
          null list in an array context.

      readlink(EXPR)

      readlink EXPR
          Returns the value of a symbolic link, if symbolic links are
          implemented.  If not, gives a fatal error.  If there is some
          system error, returns the undefined value and sets $! (errno).
          If EXPR is omitted, uses $_.

      recv(SOCKET,SCALAR,LEN,FLAGS)
          Receives a message on a socket.  Attempts to receive LENGTH
          bytes of data into variable SCALAR from the specified SOCKET
          filehandle.  Returns the address of the sender, or the
          undefined value if there's an error.  SCALAR will be grown or
          shrunk to the length actually read.  Takes the same flags as
          the system call of the same name.

      redo LABEL

      redo    The redo command restarts the loop block without evaluating
          the conditional again.  The continue block, if any, is not
          executed.  If the LABEL is omitted, the command refers to the
          innermost enclosing loop.  This command is normally used by
          programs that want to lie to themselves about what was just
          input:

           # a simpleminded Pascal comment stripper
           # (warning: assumes no { or } in strings)
           line: while (<STDIN>) {
            while (s|({.*}.*){.*}|$1 |) {}
            s|{.*}| |;
            if (s|{.*| |) {
                 $front = $_;
                 while (<STDIN>) {
                  if (/}/) {     # end of comment?
                       s|^|$front{|;
                       redo line;
                  }
                 }
            }
            print;
           }


      rename(OLDNAME,NEWNAME)
          Changes the name of a file.  Returns 1 for success, 0
          otherwise.  Will not work across filesystem boundaries.

      require(EXPR)

      require EXPR

      require Includes the library file specified by EXPR, or by $_ if EXPR
          is not supplied.    Has semantics similar to the following
          subroutine:

           sub require {
               local($filename) = @_;
               return 1 if $INC{$filename};
               local($realfilename,$result);
               ITER: {
            foreach $prefix (@INC) {
                $realfilename = "$prefix/$filename";
                if (-f $realfilename) {
                 $result = do $realfilename;
                 last ITER;
                }
            }
            die "Can't find $filename in \@INC";
               }
               die $@ if $@;
               die "$filename did not return true value" unless $result;
               $INC{$filename} = $realfilename;
               $result;
           }

          Note that the file will not be included twice under the same
          specified name.  The file must return true as the last
          statement to indicate successful execution of any
          initialization code, so it's customary to end such a file with
          "1;" unless you're sure it'll return true otherwise.

      reset(EXPR)

      reset EXPR

      reset   Generally used in a continue block at the end of a loop to
          clear variables and reset ?? searches so that they work again.
          The expression is interpreted as a list of single characters
          (hyphens allowed for ranges).  All variables and arrays
          beginning with one of those letters are reset to their
          pristine state.  If the expression is omitted, one-match
          searches (?pattern?) are reset to match again.  Only resets
          variables or searches in the current package.  Always returns
          1.  Examples:

          reset 'X';      # reset all X variables
          reset 'a-z';      # reset lower case variables
          reset;      # just reset ?? searches

          Note: resetting "A-Z" is not recommended since you'll wipe out
          your ARGV and ENV arrays.

          The use of reset on dbm associative arrays does not change the
          dbm file.  (It does, however, flush any entries cached by
          perl, which may be useful if you are sharing the dbm file.
          Then again, maybe not.)

      return LIST
          Returns from a subroutine with the value specified.  (Note
          that a subroutine can automatically return the value of the
          last expression evaluated.  That's the preferred method--use
          of an explicit return is a bit slower.)

      reverse(LIST)

      reverse LIST
          In an array context, returns an array value consisting of the
          elements of LIST in the opposite order.  In a scalar context,
          returns a string value consisting of the bytes of the first
          element of LIST in the opposite order.

      rewinddir(DIRHANDLE)

      rewinddir DIRHANDLE
          Sets the current position to the beginning of the directory
          for the readdir() routine on DIRHANDLE.

      rindex(STR,SUBSTR,POSITION)

      rindex(STR,SUBSTR)
          Works just like index except that it returns the position of
          the LAST occurrence of SUBSTR in STR.  If POSITION is
          specified, returns the last occurrence at or before that
          position.

      rmdir(FILENAME)

      rmdir FILENAME
          Deletes the directory specified by FILENAME if it is empty.
          If it succeeds it returns 1, otherwise it returns 0 and sets
          $! (errno).  If FILENAME is omitted, uses $_.

      s/PATTERN/REPLACEMENT/gieo
          Searches a string for a pattern, and if found, replaces that
          pattern with the replacement text and returns the number of
          substitutions made.  Otherwise it returns false (0).  The "g"
          is optional, and if present, indicates that all occurrences of
          the pattern are to be replaced.  The "i" is also optional, and
          if present, indicates that matching is to be done in a case-
          insensitive manner.  The "e" is likewise optional, and if
          present, indicates that the replacement string is to be
          evaluated as an expression rather than just as a double-quoted
          string.  Any non-alphanumeric delimiter may replace the
          slashes; if single quotes are used, no interpretation is done
          on the replacement string (the e modifier overrides this,
          however); if backquotes are used, the replacement string is a
          command to execute whose output will be used as the actual
          replacement text.  If the PATTERN is delimited by bracketing
          quotes, the REPLACEMENT has its own pair of quotes, which may
          or may not be bracketing quotes, e.g.  s(foo)(bar) or
          s<foo>/bar/.  If no string is specified via the =~ or !~
          operator, the $_ string is searched and modified.  (The string
          specified with =~ must be a scalar variable, an array element,
          or an assignment to one of those, i.e. an lvalue.) If the
          pattern contains a $ that looks like a variable rather than an
          end-of-string test, the variable will be interpolated into the
          pattern at run-time.  If you only want the pattern compiled
          once the first time the variable is interpolated, add an "o"
          at the end.  If the PATTERN evaluates to a null string, the
          most recent successful regular expression is used instead.
          See also the section on regular expressions.  Examples:

          s/\bgreen\b/mauve/g;        # don't change wintergreen

          $path =~ s|/usr/bin|/usr/local/bin|;

          s/Login: $foo/Login: $bar/; # run-time pattern

          ($foo = $bar) =~ s/bar/foo/;

          $_ = 'abc123xyz';
          s/\d+/$&*2/e;        # yields 'abc246xyz'
          s/\d+/sprintf("%5d",$&)/e;     # yields 'abc    246xyz'
          s/\w/$& x 2/eg;      # yields 'aabbcc  224466xxyyzz'

          s/([^ ]*) *([^ ]*)/$2 $1/;     # reverse 1st two fields

          (Note the use of $ instead of \ in the last example.  See
          section on regular expressions.)

      scalar(EXPR)
          Forces EXPR to be interpreted in a scalar context and returns
          the value of EXPR.

      seek(FILEHANDLE,POSITION,WHENCE)
          Randomly positions the file pointer for FILEHANDLE, just like
          the fseek() call of stdio.  FILEHANDLE may be an expression
          whose value gives the name of the filehandle.  Returns 1 upon
          success, 0 otherwise.

      seekdir(DIRHANDLE,POS)
          Sets the current position for the readdir() routine on
          DIRHANDLE.  POS must be a value returned by telldir().  Has
          the same caveats about possible directory compaction as the
          corresponding system library routine.

      select(FILEHANDLE)

      select  Returns the currently selected filehandle.  Sets the current
          default filehandle for output, if FILEHANDLE is supplied.
          This has two effects: first, a write or a print without a
          filehandle will default to this FILEHANDLE.  Second,
          references to variables related to output will refer to this
          output channel.  For example, if you have to set the top of
          form format for more than one output channel, you might do the
          following:




           select(REPORT1);
           $^ = 'report1_top';
           select(REPORT2);
           $^ = 'report2_top';

          FILEHANDLE may be an expression whose value gives the name of
          the actual filehandle.  Thus:

           $oldfh = select(STDERR); $| = 1; select($oldfh);


      select(RBITS,WBITS,EBITS,TIMEOUT)
          This calls the select system call with the bitmasks specified,
          which can be constructed using fileno() and vec(), along these
          lines:

           $rin = $win = $ein = '';
           vec($rin,fileno(STDIN),1) = 1;
           vec($win,fileno(STDOUT),1) = 1;
           $ein = $rin | $win;

          If you want to select on many filehandles you might wish to
          write a subroutine:

           sub fhbits {
               local(@fhlist) = split(' ',$_[0]);
               local($bits);
               for (@fhlist) {
            vec($bits,fileno($_),1) = 1;
               }
               $bits;
           }
           $rin = &fhbits('STDIN TTY SOCK');

          The usual idiom is:

           ($nfound,$timeleft) =
             select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

          or to block until something becomes ready:

           $nfound = select($rout=$rin, $wout=$win,
                  $eout=$ein, undef);

          Any of the bitmasks can also be undef.  The timeout, if
          specified, is in seconds, which may be fractional.  NOTE: not
          all implementations are capable of returning the $timeleft.
          If not, they always return $timeleft equal to the supplied
          $timeout.

      semctl(ID,SEMNUM,CMD,ARG)
          Calls the System V IPC function semctl.  If CMD is &IPC_STAT
          or &GETALL, then ARG must be a variable which will hold the
          returned semid_ds structure or semaphore value array.  Returns
          like ioctl: the undefined value for error, "0 but true" for
          zero, or the actual return value otherwise.

      semget(KEY,NSEMS,SIZE,FLAGS)
          Calls the System V IPC function semget.  Returns the semaphore
          id, or the undefined value if there is an error.

      semop(KEY,OPSTRING)
          Calls the System V IPC function semop to perform semaphore
          operations such as signaling and waiting.  OPSTRING must be a
          packed array of semop structures.  Each semop structure can be
          generated with 'pack("sss", $semnum, $semop, $semflag)'.    The
          number of semaphore operations is implied by the length of
          OPSTRING.  Returns true if successful, or false if there is an
          error.  As an example, the following code waits on semaphore
          $semnum of semaphore id $semid:

           $semop = pack("sss", $semnum, -1, 0);
           die "Semaphore trouble: $!\n" unless semop($semid, $semop);

          To signal the semaphore, replace "-1" with "1".

      send(SOCKET,MSG,FLAGS,TO)

      send(SOCKET,MSG,FLAGS)
          Sends a message on a socket.  Takes the same flags as the
          system call of the same name.  On unconnected sockets you must
          specify a destination to send TO.  Returns the number of
          characters sent, or the undefined value if there is an error.

      setpgrp(PID,PGRP)
          Sets the current process group for the specified PID, 0 for
          the current process.  Will produce a fatal error if used on a
          machine that doesn't implement setpgrp(2).

      setpriority(WHICH,WHO,PRIORITY)
          Sets the current priority for a process, a process group, or a
          user.  (See setpriority(2).) Will produce a fatal error if
          used on a machine that doesn't implement setpriority(2).

      setsockopt(SOCKET,LEVEL,OPTNAME,OPTVAL)
          Sets the socket option requested.  Returns undefined if there
          is an error.  OPTVAL may be specified as undef if you don't
          want to pass an argument.

      shift(ARRAY)

      shift ARRAY

      shift   Shifts the first value of the array off and returns it,
          shortening the array by 1 and moving everything down.  If
          there are no elements in the array, returns the undefined
          value.  If ARRAY is omitted, shifts the @ARGV array in the
          main program, and the @_ array in subroutines.  (This is
          determined lexically.) See also unshift(), push() and pop().
          Shift() and unshift() do the same thing to the left end of an
          array that push() and pop() do to the right end.

      shmctl(ID,CMD,ARG)
          Calls the System V IPC function shmctl.  If CMD is &IPC_STAT,
          then ARG must be a variable which will hold the returned
          shmid_ds structure.  Returns like ioctl: the undefined value
          for error, "0 but true" for zero, or the actual return value
          otherwise.

      shmget(KEY,SIZE,FLAGS)
          Calls the System V IPC function shmget.  Returns the shared
          memory segment id, or the undefined value if there is an
          error.

      shmread(ID,VAR,POS,SIZE)

      shmwrite(ID,STRING,POS,SIZE)
          Reads or writes the System V shared memory segment ID starting
          at position POS for size SIZE by attaching to it, copying
          in/out, and detaching from it.  When reading, VAR must be a
          variable which will hold the data read.  When writing, if
          STRING is too long, only SIZE bytes are used; if STRING is too
          short, nulls are written to fill out SIZE bytes.    Return true
          if successful, or false if there is an error.

      shutdown(SOCKET,HOW)
          Shuts down a socket connection in the manner indicated by HOW,
          which has the same interpretation as in the system call of the
          same name.

      sin(EXPR)

      sin EXPR
          Returns the sine of EXPR (expressed in radians).    If EXPR is
          omitted, returns sine of $_.

      sleep(EXPR)

      sleep EXPR

      sleep   Causes the script to sleep for EXPR seconds, or forever if no
          EXPR.  May be interrupted by sending the process a SIGALRM.
          Returns the number of seconds actually slept.  You probably
          cannot mix alarm() and sleep() calls, since sleep() is often
          implemented using alarm().

      socket(SOCKET,DOMAIN,TYPE,PROTOCOL)
          Opens a socket of the specified kind and attaches it to
          filehandle SOCKET.  DOMAIN, TYPE and PROTOCOL are specified
          the same as for the system call of the same name.  You may
          need to run h2ph on sys/socket.h to get the proper values
          handy in a perl library file.  Return true if successful.  See
          the example in the section on Interprocess Communication.

      socketpair(SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL)
          Creates an unnamed pair of sockets in the specified domain, of
          the specified type.  DOMAIN, TYPE and PROTOCOL are specified
          the same as for the system call of the same name.  If
          unimplemented, yields a fatal error.  Return true if
          successful.

      sort(SUBROUTINE LIST)

      sort(LIST)

      sort SUBROUTINE LIST

      sort BLOCK LIST

      sort LIST
          Sorts the LIST and returns the sorted array value.
          Nonexistent values of arrays are stripped out.  If SUBROUTINE
          or BLOCK is omitted, sorts in standard string comparison
          order.  If SUBROUTINE is specified, gives the name of a
          subroutine that returns an integer less than, equal to, or
          greater than 0, depending on how the elements of the array are
          to be ordered.  (The <=> and cmp operators are extremely
          useful in such routines.) SUBROUTINE may be a scalar variable
          name, in which case the value provides the name of the
          subroutine to use.  In place of a SUBROUTINE name, you can
          provide a BLOCK as an anonymous, in-line sort subroutine.

          In the interests of efficiency the normal calling code for
          subroutines is bypassed, with the following effects: the
          subroutine may not be a recursive subroutine, and the two
          elements to be compared are passed into the subroutine not via
          @_ but as $a and $b (see example below).    They are passed by
          reference so don't modify $a and $b.

          Examples:

           # sort lexically
           @articles = sort @files;

           # same thing, but with explicit sort routine
           @articles = sort {$a cmp $b} @files;

           # same thing in reversed order
           @articles = sort {$b cmp $a} @files;

           # sort numerically ascending
           @articles = sort {$a <=> $b} @files;

           # sort numerically descending
           @articles = sort {$b <=> $a} @files;

           # sort using explicit subroutine name
           sub byage {
               $age{$a} <=> $age{$b};     # presuming integers
           }
           @sortedclass = sort byage @class;

           sub reverse { $b cmp $a; }
           @harry = ('dog','cat','x','Cain','Abel');
           @george = ('gone','chased','yz','Punished','Axed');
           print sort @harry;
            # prints AbelCaincatdogx
           print sort reverse @harry;
            # prints xdogcatCainAbel
           print sort @george, 'to', @harry;
            # prints AbelAxedCainPunishedcatchaseddoggonetoxyz


      splice(ARRAY,OFFSET,LENGTH,LIST)

      splice(ARRAY,OFFSET,LENGTH)

      splice(ARRAY,OFFSET)
          Removes the elements designated by OFFSET and LENGTH from an
          array, and replaces them with the elements of LIST, if any.
          Returns the elements removed from the array.  The array grows
          or shrinks as necessary.    If LENGTH is omitted, removes
          everything from OFFSET onward.  The following equivalencies
          hold (assuming $[ == 0):

           push(@a,$x,$y)         splice(@a,$#a+1,0,$x,$y)
           pop(@a)             splice(@a,-1)
           shift(@a)             splice(@a,0,1)
           unshift(@a,$x,$y)         splice(@a,0,0,$x,$y)
           $a[$x] = $y             splice(@a,$x,1,$y);

          Example, assuming array lengths are passed before arrays:

           sub aeq { # compare two array values
            local(@a) = splice(@_,0,shift);
            local(@b) = splice(@_,0,shift);
            return 0 unless @a == @b;     # same len?
            while (@a) {
                return 0 if pop(@a) ne pop(@b);
            }
            return 1;
           }
           if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }


      split(/PATTERN/,EXPR,LIMIT)

      split(/PATTERN/,EXPR)

      split(/PATTERN/)

      split   Splits a string into an array of strings, and returns it.  (If
          not in an array context, returns the number of fields found
          and splits into the @_ array.  (In an array context, you can
          force the split into @_ by using ?? as the pattern delimiters,
          but it still returns the array value.)) If EXPR is omitted,
          splits the $_ string.  If PATTERN is also omitted, splits on
          whitespace (/[ \t\n]+/).    Anything matching PATTERN is taken
          to be a delimiter separating the fields.    (Note that the
          delimiter may be longer than one character.) If LIMIT is
          specified, splits into no more than that many fields (though
          it may split into fewer).  If LIMIT is unspecified, trailing
          null fields are stripped (which potential users of pop() would
          do well to remember).  A pattern matching the null string (not
          to be confused with a null pattern //, which is just one
          member of the set of patterns matching a null string) will
          split the value of EXPR into separate characters at each point
          it matches that way.  For example:

           print join(':', split(/ */, 'hi there'));

          produces the output 'h:i:t:h:e:r:e'.

          The LIMIT parameter can be used to partially split a line

           ($login, $passwd, $remainder) = split(/:/, $_, 3);

          (When assigning to a list, if LIMIT is omitted, perl supplies
          a LIMIT one larger than the number of variables in the list,
          to avoid unnecessary work.  For the list above LIMIT would
          have been 4 by default.  In time critical applications it
          behooves you not to split into more fields than you really
          need.)

          If the PATTERN contains parentheses, additional array elements
          are created from each matching substring in the delimiter.

           split(/([,-])/,"1-10,20");

          produces the array value

           (1,'-',10,',',20)

          The pattern /PATTERN/ may be replaced with an expression to
          specify patterns that vary at runtime.  (To do runtime
          compilation only once, use /$variable/o.) As a special case,
          specifying a space (' ') will split on white space just as
          split with no arguments does, but leading white space does NOT
          produce a null first field.  Thus, split(' ') can be used to
          emulate awk's default behavior, whereas split(/ /) will give
          you as many null initial fields as there are leading spaces.

          Example:

           open(passwd, '/etc/passwd');
           while (<passwd>) {
            ($login, $passwd, $uid, $gid, $gcos, $home, $shell)
                 = split(/:/);
            ...
           }

          (Note that $shell above will still have a newline on it.    See
          chop().) See also join.

      sprintf(FORMAT,LIST)
          Returns a string formatted by the usual printf conventions.
          The * character is not supported.

      sqrt(EXPR)

      sqrt EXPR
          Return the square root of EXPR.  If EXPR is omitted, returns
          square root of $_.

      srand(EXPR)

      srand EXPR
          Sets the random number seed for the rand operator.  If EXPR is
          omitted, does srand(time).

      stat(FILEHANDLE)

      stat FILEHANDLE

      stat(EXPR)

      stat SCALARVARIABLE
          Returns a 13-element array giving the statistics for a file,
          either the file opened via FILEHANDLE, or named by EXPR.
          Returns a null list if the stat fails.  Typically used as
          follows:

          ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
             $atime,$mtime,$ctime,$blksize,$blocks)
             = stat($filename);

          If stat is passed the special filehandle consisting of an
          underline, no stat is done, but the current contents of the
          stat structure from the last stat or filetest are returned.
          Example:

           if (-x $file && (($d) = stat(_)) && $d < 0) {
            print "$file is executable NFS file\n";
           }

          (This only works on machines for which the device number is
          negative under NFS.)

      study(SCALAR)

      study SCALAR

      study   Takes extra time to study SCALAR ($_ if unspecified) in
          anticipation of doing many pattern matches on the string
          before it is next modified.  This may or may not save time,
          depending on the nature and number of patterns you are
          searching on, and on the distribution of character frequencies
          in the string to be searched--you probably want to compare
          runtimes with and without it to see which runs faster.  Those
          loops which scan for many short constant strings (including
          the constant parts of more complex patterns) will benefit
          most.  You may have only one study active at a time--if you
          study a different scalar the first is "unstudied".  (The way
          study works is this: a linked list of every character in the
          string to be searched is made, so we know, for example, where
          all the 'k' characters are.  From each search string, the
          rarest character is selected, based on some static frequency
          tables constructed from some C programs and English text.
          Only those places that contain this "rarest" character are
          examined.)

          For example, here is a loop which inserts index producing
          entries before any line containing a certain pattern:

           while (<>) {
            study;
            print ".IX foo\n" if /\bfoo\b/;
            print ".IX bar\n" if /\bbar\b/;
            print ".IX blurfl\n" if /\bblurfl\b/;
            ...
            print;
           }

          In searching for /\bfoo\b/, only those locations in $_ that
          contain 'f' will be looked at, because 'f' is rarer than 'o'.
          In general, this is a big win except in pathological cases.
          The only question is whether it saves you more time than it
          took to build the linked list in the first place.

          Note that if you have to look for strings that you don't know
          till runtime, you can build an entire loop as a string and
          eval that to avoid recompiling all your patterns all the time.
          Together with undefining $/ to input entire files as one
          record, this can be very fast, often faster than specialized
          programs like fgrep.  The following scans a list of files
          (@files) for a list of words (@words), and prints out the
          names of those files that contain a match:

           $search = 'while (<>) { study;';
           foreach $word (@words) {
               $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
           }
           $search .= "}";
           @ARGV = @files;
           undef $/;
           eval $search;       # this screams
           $/ = "\n";           # put back to normal input delim
           foreach $file (sort keys(%seen)) {
               print $file, "\n";
           }


      substr(EXPR,OFFSET,LEN)

      substr(EXPR,OFFSET)
          Extracts a substring out of EXPR and returns it.    First
          character is at offset 0, or whatever you've set $[ to.  If
          OFFSET is negative, starts that far from the end of the
          string.  If LEN is omitted, returns everything to the end of
          the string.  You can use the substr() function as an lvalue,
          in which case EXPR must be an lvalue.  If you assign something
          shorter than LEN, the string will shrink, and if you assign
          something longer than LEN, the string will grow to accommodate
          it.  To keep the string the same length you may need to pad or
          chop your value using sprintf().

      symlink(OLDFILE,NEWFILE)
          Creates a new filename symbolically linked to the old
          filename.  Returns 1 for success, 0 otherwise.  On systems
          that don't support symbolic links, produces a fatal error at
          run time.  To check for that, use eval:

           $symlink_exists = (eval 'symlink("","");', $@ eq '');


      syscall(LIST)

      syscall LIST
          Calls the system call specified as the first element of the
          list, passing the remaining elements as arguments to the
          system call.  If unimplemented, produces a fatal error.  The
          arguments are interpreted as follows: if a given argument is
          numeric, the argument is passed as an int.  If not, the
          pointer to the string value is passed.  You are responsible to
          make sure a string is pre-extended long enough to receive any
          result that might be written into a string.  If your integer
          arguments are not literals and have never been interpreted in
          a numeric context, you may need to add 0 to them to force them
          to look like numbers.

           require 'syscall.ph';     # may need to run h2ph
           syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);


      sysread(FILEHANDLE,SCALAR,LENGTH,OFFSET)

      sysread(FILEHANDLE,SCALAR,LENGTH)
          Attempts to read LENGTH bytes of data into variable SCALAR
          from the specified FILEHANDLE, using the system call read(2).
          It bypasses stdio, so mixing this with other kinds of reads
          may cause confusion.  Returns the number of bytes actually
          read, or undef if there was an error.  SCALAR will be grown or
          shrunk to the length actually read.  An OFFSET may be
          specified to place the read data at some other place than the
          beginning of the string.

      system(LIST)

      system LIST
          Does exactly the same thing as "exec LIST" except that a fork
          is done first, and the parent process waits for the child
          process to complete.  Note that argument processing varies
          depending on the number of arguments.  The return value is the
          exit status of the program as returned by the wait() call.  To
          get the actual exit value divide by 256.    See also exec.

      syswrite(FILEHANDLE,SCALAR,LENGTH,OFFSET)

      syswrite(FILEHANDLE,SCALAR,LENGTH)
          Attempts to write LENGTH bytes of data from variable SCALAR to
          the specified FILEHANDLE, using the system call write(2).  It
          bypasses stdio, so mixing this with prints may cause
          confusion.  Returns the number of bytes actually written, or
          undef if there was an error.  An OFFSET may be specified to
          place the read data at some other place than the beginning of
          the string.

      tell(FILEHANDLE)

      tell FILEHANDLE

      tell    Returns the current file position for FILEHANDLE.  FILEHANDLE
          may be an expression whose value gives the name of the actual
          filehandle.  If FILEHANDLE is omitted, assumes the file last
          read.

      telldir(DIRHANDLE)

      telldir DIRHANDLE
          Returns the current position of the readdir() routines on
          DIRHANDLE.  Value may be given to seekdir() to access a
          particular location in a directory.  Has the same caveats
          about possible directory compaction as the corresponding
          system library routine.

      time    Returns the number of non-leap seconds since 00:00:00 UTC,
          January 1, 1970.    Suitable for feeding to gmtime() and
          localtime().

      times   Returns a four-element array giving the user and system times,
          in seconds, for this process and the children of this process.

          ($user,$system,$cuser,$csystem) = times;


      tr/SEARCHLIST/REPLACEMENTLIST/cds

      y/SEARCHLIST/REPLACEMENTLIST/cds
          Translates all occurrences of the characters found in the
          search list with the corresponding character in the
          replacement list.  It returns the number of characters
          replaced or deleted.  If no string is specified via the =~ or
          !~ operator, the $_ string is translated.  (The string
          specified with =~ must be a scalar variable, an array element,
          or an assignment to one of those, i.e. an lvalue.) For sed
          devotees, y is provided as a synonym for tr.  If the
          SEARCHLIST is delimited by bracketing quotes, the
          REPLACEMENTLIST has its own pair of quotes, which may or may
          not be bracketing quotes, e.g.  tr[A-Z][a-z] or tr(+-
          */)/ABCD/.

          If the c modifier is specified, the SEARCHLIST character set
          is complemented.    If the d modifier is specified, any
          characters specified by SEARCHLIST that are not found in
          REPLACEMENTLIST are deleted.  (Note that this is slightly more
          flexible than the behavior of some tr programs, which delete
          anything they find in the SEARCHLIST, period.) If the s
          modifier is specified, sequences of characters that were
          translated to the same character are squashed down to 1
          instance of the character.

          If the d modifier was used, the REPLACEMENTLIST is always
          interpreted exactly as specified.  Otherwise, if the
          REPLACEMENTLIST is shorter than the SEARCHLIST, the final
          character is replicated till it is long enough.  If the
          REPLACEMENTLIST is null, the SEARCHLIST is replicated.  This
          latter is useful for counting characters in a class, or for
          squashing character sequences in a class.

          Examples:

          $ARGV[1] =~ y/A-Z/a-z/;   # canonicalize to lower case

          $cnt = tr/*/*/;        # count the stars in $_

          $cnt = tr/0-9//;        # count the digits in $_

          tr/a-zA-Z//s;         # bookkeeper -> bokeper

          ($HOST = $host) =~ tr/a-z/A-Z/;

          y/a-zA-Z/ /cs;        # change non-alphas to single space

          tr/\200-\377/\0-\177/;    # delete 8th bit


      truncate(FILEHANDLE,LENGTH)

      truncate(EXPR,LENGTH)
          Truncates the file opened on FILEHANDLE, or named by EXPR, to
          the specified length.  Produces a fatal error if truncate
          isn't implemented on your system.

      umask(EXPR)

      umask EXPR

      umask   Sets the umask for the process and returns the old one.  If
          EXPR is omitted, merely returns current umask.

      undef(EXPR)

      undef EXPR

      undef   Undefines the value of EXPR, which must be an lvalue.  Use
          only on a scalar value, an entire array, or a subroutine name
          (using &).  (Undef will probably not do what you expect on
          most predefined variables or dbm array values.) Always returns
          the undefined value.  You can omit the EXPR, in which case
          nothing is undefined, but you still get an undefined value
          that you could, for instance, return from a subroutine.
          Examples:

           undef $foo;
           undef $bar{'blurfl'};
           undef @ary;
           undef %assoc;
           undef &mysub;
           return (wantarray ? () : undef) if $they_blew_it;


      unlink(LIST)

      unlink LIST
          Deletes a list of files.    Returns the number of files
          successfully deleted.

           $cnt = unlink 'a', 'b', 'c';
           unlink @goners;
           unlink <*.bak>;

          Note: unlink will not delete directories unless you are
          superuser and the -U flag is supplied to perl.  Even if these
          conditions are met, be warned that unlinking a directory can
          inflict damage on your filesystem.  Use rmdir instead.

      unpack(TEMPLATE,EXPR)
          Unpack does the reverse of pack: it takes a string
          representing a structure and expands it out into an array
          value, returning the array value.  (In a scalar context, it
          merely returns the first value produced.) The TEMPLATE has the
          same format as in the pack function.  Here's a subroutine that
          does substring:

           sub substr {
            local($what,$where,$howmuch) = @_;
            unpack("x$where a$howmuch", $what);
           }

          and then there's

           sub ord { unpack("c",$_[0]); }

          In addition, you may prefix a field with a %<number> to
          indicate that you want a <number>-bit checksum of the items
          instead of the items themselves.    Default is a 16-bit
          checksum.  For example, the following computes the same number
          as the System V sum program:

           while (<>) {
               $checksum += unpack("%16C*", $_);
           }
           $checksum %= 65536;


      unshift(ARRAY,LIST)
          Does the opposite of a shift.  Or the opposite of a push,
          depending on how you look at it.    Prepends list to the front
          of the array, and returns the number of elements in the new
          array.

           unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;


      utime(LIST)

      utime LIST
          Changes the access and modification times on each file of a
          list of files.  The first two elements of the list must be the
          NUMERICAL access and modification times, in that order.
          Returns the number of files successfully changed.  The inode
          modification time of each file is set to the current time.
          Example of a "touch" command:

           #!/usr/bin/perl
           $now = time;
           utime $now, $now, @ARGV;


      values(ASSOC_ARRAY)

      values ASSOC_ARRAY
          Returns a normal array consisting of all the values of the
          named associative array.    The values are returned in an
          apparently random order, but it is the same order as either
          the keys() or each() function would produce on the same array.
          See also keys() and each().

      vec(EXPR,OFFSET,BITS)
          Treats a string as a vector of unsigned integers, and returns
          the value of the bitfield specified.  May also be assigned to.
          BITS must be a power of two from 1 to 32.

          Vectors created with vec() can also be manipulated with the
          logical operators |, & and ^, which will assume a bit vector
          operation is desired when both operands are strings.  This
          interpretation is not enabled unless there is at least one
          vec() in your program, to protect older programs.

          To transform a bit vector into a string or array of 0's and
          1's, use these:

           $bits = unpack("b*", $vector);
           @bits = split(//, unpack("b*", $vector));

          If you know the exact length in bits, it can be used in place
          of the *.

      wait    Waits for a child process to terminate and returns the pid of
          the deceased process, or -1 if there are no child processes.
          The status is returned in $?.

      waitpid(PID,FLAGS)
          Waits for a particular child process to terminate and returns
          the pid of the deceased process, or -1 if there is no such
          child process.  The status is returned in $?.  If you say

           require "sys/wait.h";
           ...
           waitpid(-1,&WNOHANG);

          then you can do a non-blocking wait for any process.  Non-
          blocking wait is only available on machines supporting either
          the waitpid (2) or wait4 (2) system calls.  However, waiting
          for a particular pid with FLAGS of 0 is implemented
          everywhere.  (Perl emulates the system call by remembering the
          status values of processes that have exited but have not been
          harvested by the Perl script yet.)

      wantarray
          Returns true if the context of the currently executing
          subroutine is looking for an array value.  Returns false if
          the context is looking for a scalar.

           return wantarray ? () : undef;

      warn(LIST)

      warn LIST
          Produces a message on STDERR just like "die", but doesn't
          exit.

      write(FILEHANDLE)

      write(EXPR)

      write   Writes a formatted record (possibly multi-line) to the
          specified file, using the format associated with that file.
          By default the format for a file is the one having the same
          name is the filehandle, but the format for the current output
          channel (see select) may be set explicitly by assigning the
          name of the format to the $~ variable.

          Top of form processing is handled automatically: if there is
          insufficient room on the current page for the formatted
          record, the page is advanced by writing a form feed, a special
          top-of-page format is used to format the new page header, and
          then the record is written.  By default the top-of-page format
          is the name of the filehandle with "_TOP" appended, but it may
          be dynamicallly set to the format of your choice by assigning
          the name to the $^ variable while the filehandle is selected.
          The number of lines remaining on the current page is in
          variable $-, which can be set to 0 to force a new page.

          If FILEHANDLE is unspecified, output goes to the current
          default output channel, which starts out as STDOUT but may be
          changed by the select operator.  If the FILEHANDLE is an EXPR,
          then the expression is evaluated and the resulting string is
          used to look up the name of the FILEHANDLE at run time.  For
          more on formats, see the section on formats later on.

          Note that write is NOT the opposite of read.

      Precedence

      Perl operators have the following associativity and precedence:

      nonassoc    print printf exec system sort reverse
             chmod chown kill unlink utime die return
      left    ,
      right    = += -= *= etc.
      right    ?:
      nonassoc    ..
      left    ||
      left    &&
      left    | ^
      left    &
      nonassoc    == != <=> eq ne cmp
      nonassoc    < > <= >= lt gt le ge
      nonassoc    chdir exit eval reset sleep rand umask
      nonassoc    -r -w -x etc.
      left    << >>
      left    + - .
      left    * / % x
      left    =~ !~
      right    ! ~ and unary minus
      right    **
      nonassoc    ++ --
      left    '('

      As mentioned earlier, if any list operator (print, etc.) or any unary
      operator (chdir, etc.) is followed by a left parenthesis as the next
      token on the same line, the operator and arguments within parentheses
      are taken to be of highest precedence, just like a normal function
      call.  Examples:

       chdir $foo || die;        # (chdir $foo) || die
       chdir($foo) || die;        # (chdir $foo) || die
       chdir ($foo) || die;     # (chdir $foo) || die
       chdir +($foo) || die;    # (chdir $foo) || die

      but, because * is higher precedence than ||:

       chdir $foo * 20;        # chdir ($foo * 20)
       chdir($foo) * 20;        # (chdir $foo) * 20
       chdir ($foo) * 20;        # (chdir $foo) * 20
       chdir +($foo) * 20;        # chdir ($foo * 20)

       rand 10 * 20;        # rand (10 * 20)
       rand(10) * 20;        # (rand 10) * 20
       rand (10) * 20;        # (rand 10) * 20
       rand +(10) * 20;        # rand (10 * 20)

      In the absence of parentheses, the precedence of list operators such
      as print, sort or chmod is either very high or very low depending on
      whether you look at the left side of operator or the right side of it.
      For example, in

       @ary = (1, 3, sort 4, 2);
       print @ary;           # prints 1324

      the commas on the right of the sort are evaluated before the sort, but
      the commas on the left are evaluated after.  In other words, list
      operators tend to gobble up all the arguments that follow them, and
      then act like a simple term with regard to the preceding expression.
      Note that you have to be careful with parens:

       # These evaluate exit before doing the print:
       print($foo, exit);  # Obviously not what you want.
       print $foo, exit;   # Nor is this.

       # These do the print before evaluating exit:
       (print $foo), exit; # This is what you want.
       print($foo), exit;  # Or this.
       print ($foo), exit; # Or even this.

      Also note that

       print ($foo & 255) + 1, "\n";

      probably doesn't do what you expect at first glance.

      Subroutines

      A subroutine may be declared as follows:

      sub NAME BLOCK


      Any arguments passed to the routine come in as array @_, that is
      ($_[0], $_[1], ...).  The array @_ is a local array, but its values
      are references to the actual scalar parameters.  The return value of
      the subroutine is the value of the last expression evaluated, and can
      be either an array value or a scalar value.  Alternately, a return
      statement may be used to specify the returned value and exit the
      subroutine.  To create local variables see the local operator.

      A subroutine is called using the do operator or the & operator.

      Example:

       sub MAX {
        local($max) = pop(@_);
        foreach $foo (@_) {
             $max = $foo if $max < $foo;
        }
        $max;
       }

       ...
       $bestday = &MAX($mon,$tue,$wed,$thu,$fri);

      Example:

       # get a line, combining continuation lines
       #  that start with whitespace
       sub get_line {
        $thisline = $lookahead;
        line: while ($lookahead = <STDIN>) {
             if ($lookahead =~ /^[ \t]/) {
              $thisline .= $lookahead;
             }
             else {
              last line;
             }
        }
        $thisline;
       }

       $lookahead = <STDIN>;    # get first line
       while ($_ = do get_line()) {
        ...
       }

      Use array assignment to a local list to name your formal arguments:

       sub maybeset {
        local($key, $value) = @_;
        $foo{$key} = $value unless $foo{$key};
       }

      This also has the effect of turning call-by-reference into call-by-
      value, since the assignment copies the values.

      Subroutines may be called recursively.  If a subroutine is called
      using the & form, the argument list is optional.    If omitted, no @_
      array is set up for the subroutine; the @_ array at the time of the
      call is visible to subroutine instead.

       do foo(1,2,3);      # pass three arguments
       &foo(1,2,3);        # the same

       do foo();      # pass a null list
       &foo();           # the same
       &foo;           # pass no arguments--more efficient


      Passing By Reference

      Sometimes you don't want to pass the value of an array to a subroutine
      but rather the name of it, so that the subroutine can modify the
      global copy of it rather than working with a local copy.    In perl you
      can refer to all the objects of a particular name by prefixing the
      name with a star: *foo.  When evaluated, it produces a scalar value
      that represents all the objects of that name, including any
      filehandle, format or subroutine.  When assigned to within a local()
      operation, it causes the name mentioned to refer to whatever * value
      was assigned to it.  Example:

       sub doubleary {
           local(*someary) = @_;
           foreach $elem (@someary) {
        $elem *= 2;
           }
       }
       do doubleary(*foo);
       do doubleary(*bar);

      Assignment to *name is currently recommended only inside a local().
      You can actually assign to *name anywhere, but the previous referent
      of *name may be stranded forever.  This may or may not bother you.

      Note that scalars are already passed by reference, so you can modify
      scalar arguments without using this mechanism by referring explicitly
      to the $_[nnn] in question.  You can modify all the elements of an
      array by passing all the elements as scalars, but you have to use the
      * mechanism to push, pop or change the size of an array.    The *
      mechanism will probably be more efficient in any case.

      Since a *name value contains unprintable binary data, if it is used as
      an argument in a print, or as a %s argument in a printf or sprintf, it
      then has the value '*name', just so it prints out pretty.

      Even if you don't want to modify an array, this mechanism is useful
      for passing multiple arrays in a single LIST, since normally the LIST
      mechanism will merge all the array values so that you can't extract
      out the individual arrays.

      Regular Expressions

      The patterns used in pattern matching are regular expressions such as
      those supplied in the Version 8 regexp routines.    (In fact, the
      routines are derived from Henry Spencer's freely redistributable
      reimplementation of the V8 routines.) In addition, \w matches an
      alphanumeric character (including "_") and \W a nonalphanumeric.    Word
      boundaries may be matched by \b, and non-boundaries by \B.  A
      whitespace character is matched by \s, non-whitespace by \S.  A
      numeric character is matched by \d, non-numeric by \D.  You may use
      \w, \s and \d within character classes.  Also, \n, \r, \f, \t and \NNN
      have their normal interpretations.  Within character classes \b
      represents backspace rather than a word boundary.  Alternatives may be
      separated by |.  The bracketing construct ( ... ) may also be used, in
      which case \<digit> matches the digit'th substring.  (Outside of the
      pattern, always use $ instead of \ in front of the digit.  The scope
      of $<digit> (and $`, $& and $') extends to the end of the enclosing
      BLOCK or eval string, or to the next pattern match with
      subexpressions.  The \<digit> notation sometimes works outside the
      current pattern, but should not be relied upon.) You may have as many
      parentheses as you wish.    If you have more than 9 substrings, the
      variables $10, $11, ... refer to the corresponding substring.  Within
      the pattern, \10, \11, etc. refer back to substrings if there have
      been at least that many left parens before the backreference.
      Otherwise (for backward compatibilty) \10 is the same as \010, a
      backspace, and \11 the same as \011, a tab.  And so on.  (\1 through
      \9 are always backreferences.)

      $+ returns whatever the last bracket match matched.  $& returns the
      entire matched string.  ($0 used to return the same thing, but not any
      more.) $` returns everything before the matched string.  $' returns
      everything after the matched string.  Examples:

       s/^([^ ]*) *([^ ]*)/$2 $1/;     # swap first two words

       if (/Time: (..):(..):(..)/) {
        $hours = $1;
        $minutes = $2;
        $seconds = $3;
       }

      By default, the ^ character is only guaranteed to match at the
      beginning of the string, the $ character only at the end (or before
      the newline at the end) and perl does certain optimizations with the
      assumption that the string contains only one line.  The behavior of ^
      and $ on embedded newlines will be inconsistent.    You may, however,
      wish to treat a string as a multi-line buffer, such that the ^ will
      match after any newline within the string, and $ will match before any
      newline.    At the cost of a little more overhead, you can do this by
      setting the variable $* to 1.  Setting it back to 0 makes perl revert
      to its old behavior.

      To facilitate multi-line substitutions, the . character never matches
      a newline (even when $* is 0).  In particular, the following leaves a
      newline on the $_ string:

       $_ = <STDIN>;
       s/.*(some_string).*/$1/;

      If the newline is unwanted, try one of

       s/.*(some_string).*\n/$1/;
       s/.*(some_string)[^\000]*/$1/;
       s/.*(some_string)(.|\n)*/$1/;
       chop; s/.*(some_string).*/$1/;
       /(some_string)/ && ($_ = $1);

      Any item of a regular expression may be followed with digits in curly
      brackets of the form {n,m}, where n gives the minimum number of times
      to match the item and m gives the maximum.  The form {n} is equivalent
      to {n,n} and matches exactly n times.  The form {n,} matches n or more
      times.  (If a curly bracket occurs in any other context, it is treated
      as a regular character.) The * modifier is equivalent to {0,}, the +
      modifier to {1,} and the ? modifier to {0,1}.  There is no limit to
      the size of n or m, but large numbers will chew up more memory.

      You will note that all backslashed metacharacters in perl are
      alphanumeric, such as \b, \w, \n.  Unlike some other regular
      expression languages, there are no backslashed symbols that aren't
      alphanumeric.  So anything that looks like \\, \(, \), \<, \>, \{, or
      \} is always interpreted as a literal character, not a metacharacter.
      This makes it simple to quote a string that you want to use for a
      pattern but that you are afraid might contain metacharacters.  Simply
      quote all the non-alphanumeric characters:

       $pattern =~ s/(\W)/\\$1/g;


      Formats

      Output record formats for use with the write operator may declared as
      follows:

      format NAME =
      FORMLIST
      .

      If name is omitted, format "STDOUT" is defined.  FORMLIST consists of
      a sequence of lines, each of which may be of one of three types:

      1.  A comment.

      2.  A "picture" line giving the format for one output line.

      3.  An argument line supplying values to plug into a picture line.

      Picture lines are printed exactly as they look, except for certain
      fields that substitute values into the line.  Each picture field
      starts with either @ or ^.  The @ field (not to be confused with the
      array marker @) is the normal case; ^ fields are used to do
      rudimentary multi-line text block filling.  The length of the field is
      supplied by padding out the field with multiple <, >, or | characters
      to specify, respectively, left justification, right justification, or
      centering.  As an alternate form of right justification, you may also
      use # characters (with an optional .) to specify a numeric field.
      (Use of ^ instead of @ causes the field to be blanked if undefined.)
      If any of the values supplied for these fields contains a newline,
      only the text up to the newline is printed.  The special field @* can
      be used for printing multi-line values.  It should appear by itself on
      a line.

      The values are specified on the following line, in the same order as
      the picture fields.  The values should be separated by commas.

      Picture fields that begin with ^ rather than @ are treated specially.
      The value supplied must be a scalar variable name which contains a
      text string.  Perl puts as much text as it can into the field, and
      then chops off the front of the string so that the next time the
      variable is referenced, more of the text can be printed.    Normally you
      would use a sequence of fields in a vertical stack to print out a
      block of text.  If you like, you can end the final field with ...,
      which will appear in the output if the text was too long to appear in
      its entirety.  You can change which characters are legal to break on
      by changing the variable $: to a list of the desired characters.

      Since use of ^ fields can produce variable length records if the text
      to be formatted is short, you can suppress blank lines by putting the
      tilde (~) character anywhere in the line.  (Normally you should put it
      in the front if possible, for visibility.) The tilde will be
      translated to a space upon output.  If you put a second tilde
      contiguous to the first, the line will be repeated until all the
      fields on the line are exhausted.  (If you use a field of the @
      variety, the expression you supply had better not give the same value
      every time forever!)

      Examples:

      # a report on the /etc/passwd file
      format STDOUT_TOP =
                  Passwd File
      Name          Login    Office   Uid   Gid Home
      ------------------------------------------------------------------
      .
      format STDOUT =
      @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
      $name,          $login,  $office,$uid,$gid, $home
      .












      # a report from a bug report form
      format STDOUT_TOP =
                  Bug Reports
      @<<<<<<<<<<<<<<<<<<<<<<<       @|||     @>>>>>>>>>>>>>>>>>>>>>>>
      $system,                $%,     $date
      ------------------------------------------------------------------
      .
      format STDOUT =
      Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
           $subject
      Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
         $index,               $description
      Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        $priority,      $date,   $description
      From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        $from,               $description
      Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
           $programmer,        $description
      ~                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                       $description
      ~                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                       $description
      ~                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                       $description
      ~                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                       $description
      ~                    ^<<<<<<<<<<<<<<<<<<<<<<<...
                       $description
      .

      It is possible to intermix prints with writes on the same output
      channel, but you'll have to handle $- (lines left on the page)
      yourself.

      If you are printing lots of fields that are usually blank, you should
      consider using the reset operator between records.  Not only is it
      more efficient, but it can prevent the bug of adding another field and
      forgetting to zero it.

      Interprocess Communication

      The IPC facilities of perl are built on the Berkeley socket mechanism.
      If you don't have sockets, you can ignore this section.  The calls
      have the same names as the corresponding system calls, but the
      arguments tend to differ, for two reasons.  First, perl file handles
      work differently than C file descriptors.  Second, perl already knows
      the length of its strings, so you don't need to pass that information.
      Here is a sample client (untested):

       ($them,$port) = @ARGV;
       $port = 2345 unless $port;
       $them = 'localhost' unless $them;

       $SIG{'INT'} = 'dokill';
       sub dokill { kill 9,$child if $child; }

       require 'sys/socket.ph';

       $sockaddr = 'S n a4 x8';
       chop($hostname = `hostname`);

       ($name, $aliases, $proto) = getprotobyname('tcp');
       ($name, $aliases, $port) = getservbyname($port, 'tcp')
        unless $port =~ /^\d+$/;
       ($name, $aliases, $type, $len, $thisaddr) =
                   gethostbyname($hostname);
       ($name, $aliases, $type, $len, $thataddr) = gethostbyname($them);

       $this = pack($sockaddr, &AF_INET, 0, $thisaddr);
       $that = pack($sockaddr, &AF_INET, $port, $thataddr);

       socket(S, &PF_INET, &SOCK_STREAM, $proto) || die "socket: $!";
       bind(S, $this) || die "bind: $!";
       connect(S, $that) || die "connect: $!";

       select(S); $| = 1; select(stdout);

       if ($child = fork) {
        while (<>) {
             print S;
        }
        sleep 3;
        do dokill();
       }
       else {
        while (<S>) {
             print;
        }
       }

      And here's a server:

       ($port) = @ARGV;
       $port = 2345 unless $port;

       require 'sys/socket.ph';

       $sockaddr = 'S n a4 x8';

       ($name, $aliases, $proto) = getprotobyname('tcp');
       ($name, $aliases, $port) = getservbyname($port, 'tcp')
        unless $port =~ /^\d+$/;
       $this = pack($sockaddr, &AF_INET, $port, "\0\0\0\0");

       select(NS); $| = 1; select(stdout);

       socket(S, &PF_INET, &SOCK_STREAM, $proto) || die "socket: $!";
       bind(S, $this) || die "bind: $!";
       listen(S, 5) || die "connect: $!";

       select(S); $| = 1; select(stdout);

       for (;;) {
        print "Listening again\n";
        ($addr = accept(NS,S)) || die $!;
        print "accept ok\n";

        ($af,$port,$inetaddr) = unpack($sockaddr,$addr);
        @inetaddr = unpack('C4',$inetaddr);
        print "$af $port @inetaddr\n";

        while (<NS>) {
             print;
             print NS;
        }
       }


      Predefined Names

      The following names have special meaning to perl.  I could have used
      alphabetic symbols for some of these, but I didn't want to take the
      chance that someone would say reset "a-zA-Z" and wipe them all out.
      You'll just have to suffer along with these silly symbols.  Most of
      them have reasonable mnemonics, or analogues in one of the shells.

      $_      The default input and pattern-searching space.  The following
          pairs are equivalent:

           while (<>) {...     # only equivalent in while!
           while ($_ = <>) {...

           /^Subject:/
           $_ =~ /^Subject:/

           y/a-z/A-Z/
           $_ =~ y/a-z/A-Z/

           chop
           chop($_)

          (Mnemonic: underline is understood in certain operations.)

      $.      The current input line number of the last filehandle that was
          read.  Readonly.    Remember that only an explicit close on the
          filehandle resets the line number.  Since <> never does an
          explicit close, line numbers increase across ARGV files (but
          see examples under eof).    (Mnemonic: many programs use . to
          mean the current line number.)

      $/      The input record separator, newline by default.  Works like
          awk's RS variable, including treating blank lines as
          delimiters if set to the null string.  You may set it to a
          multicharacter string to match a multi-character delimiter.
          Note that setting it to "\n\n" means something slightly
          different than setting it to "", if the file contains
          consecutive blank lines.    Setting it to "" will treat two or
          more consecutive blank lines as a single blank line.  Setting
          it to "\n\n" will blindly assume that the next input character
          belongs to the next paragraph, even if it's a newline.
          (Mnemonic: / is used to delimit line boundaries when quoting
          poetry.)

      $,      The output field separator for the print operator.  Ordinarily
          the print operator simply prints out the comma separated
          fields you specify.  In order to get behavior more like awk,
          set this variable as you would set awk's OFS variable to
          specify what is printed between fields.  (Mnemonic: what is
          printed when there is a , in your print statement.)

      $"      This is like $, except that it applies to array values
          interpolated into a double-quoted string (or similar
          interpreted string).  Default is a space.  (Mnemonic: obvious,
          I think.)

      $\      The output record separator for the print operator.
          Ordinarily the print operator simply prints out the comma
          separated fields you specify, with no trailing newline or
          record separator assumed.  In order to get behavior more like
          awk, set this variable as you would set awk's ORS variable to
          specify what is printed at the end of the print.    (Mnemonic:
          you set $\ instead of adding \n at the end of the print.
          Also, it's just like /, but it's what you get "back" from
          perl.)

      $#      The output format for printed numbers.  This variable is a
          half-hearted attempt to emulate awk's OFMT variable.  There
          are times, however, when awk and perl have differing notions
          of what is in fact numeric.  Also, the initial value is %.20g
          rather than %.6g, so you need to set $# explicitly to get
          awk's value.  (Mnemonic: # is the number sign.)

      $%      The current page number of the currently selected output
          channel.    (Mnemonic: % is page number in nroff.)

      $=      The current page length (printable lines) of the currently
          selected output channel.    Default is 60.    (Mnemonic: = has
          horizontal lines.)

      $-      The number of lines left on the page of the currently selected
          output channel.  (Mnemonic: lines_on_page - lines_printed.)

      $~      The name of the current report format for the currently
          selected output channel.    Default is name of the filehandle.
          (Mnemonic: brother to $^.)

      $^      The name of the current top-of-page format for the currently
          selected output channel.    Default is name of the filehandle
          with "_TOP" appended.  (Mnemonic: points to top of page.)

      $|      If set to nonzero, forces a flush after every write or print
          on the currently selected output channel.  Default is 0.    Note
          that STDOUT will typically be line buffered if output is to
          the terminal and block buffered otherwise.  Setting this
          variable is useful primarily when you are outputting to a
          pipe, such as when you are running a perl script under rsh and
          want to see the output as it's happening.  (Mnemonic: when you
          want your pipes to be piping hot.)

      $$      The process number of the perl running this script.
          (Mnemonic: same as shells.)

      $?      The status returned by the last pipe close, backtick (``)
          command or system operator.  Note that this is the status word
          returned by the wait() system call, so the exit value of the
          subprocess is actually ($? >> 8).  $? & 255 gives which
          signal, if any, the process died from, and whether there was a
          core dump.  (Mnemonic: similar to sh and ksh.)

      $&      The string matched by the last successful pattern match (not
          counting any matches hidden within a BLOCK or eval enclosed by
          the current BLOCK).  (Mnemonic: like & in some editors.)

      $`      The string preceding whatever was matched by the last
          successful pattern match (not counting any matches hidden
          within a BLOCK or eval enclosed by the current BLOCK).
          (Mnemonic: ` often precedes a quoted string.)

      $'      The string following whatever was matched by the last
          successful pattern match (not counting any matches hidden
          within a BLOCK or eval enclosed by the current BLOCK).
          (Mnemonic: ' often follows a quoted string.) Example:

           $_ = 'abcdefghi';
           /def/;
           print "$`:$&:$'\n";        # prints abc:def:ghi


      $+      The last bracket matched by the last search pattern.  This is
          useful if you don't know which of a set of alternative
          patterns matched.  For example:

          /Version: (.*)|Revision: (.*)/ && ($rev = $+);

          (Mnemonic: be positive and forward looking.)

      $*      Set to 1 to do multiline matching within a string, 0 to tell
          perl that it can assume that strings contain a single line,
          for the purpose of optimizing pattern matches.  Pattern
          matches on strings containing multiple newlines can produce
          confusing results when $* is 0.  Default is 0.  (Mnemonic: *
          matches multiple things.) Note that this variable only
          influences the interpretation of ^ and $.  A literal newline
          can be searched for even when $* == 0.

      $0      Contains the name of the file containing the perl script being
          executed.  Assigning to $0 modifies the argument area that the
          ps(1) program sees.  (Mnemonic: same as sh and ksh.)

      $<digit>
          Contains the subpattern from the corresponding set of
          parentheses in the last pattern matched, not counting patterns
          matched in nested blocks that have been exited already.
          (Mnemonic: like \digit.)

      $[      The index of the first element in an array, and of the first
          character in a substring.  Default is 0, but you could set it
          to 1 to make perl behave more like awk (or Fortran) when
          subscripting and when evaluating the index() and substr()
          functions.  (Mnemonic: [ begins subscripts.)

      $]      The string printed out when you say "perl -v".  It can be used
          to determine at the beginning of a script whether the perl
          interpreter executing the script is in the right range of
          versions.  If used in a numeric context, returns the version +
          patchlevel / 1000.  Example:








           # see if getc is available
              ($version,$patchlevel) =
             $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
              print STDERR "(No filename completion available.)\n"
             if $version * 1000 + $patchlevel < 2016;

          or, used numerically,

           warn "No checksumming!\n" if $] < 3.019;

          (Mnemonic: Is this version of perl in the right bracket?)

      $;      The subscript separator for multi-dimensional array emulation.
          If you refer to an associative array element as
           $foo{$a,$b,$c}

          it really means

           $foo{join($;, $a, $b, $c)}

          But don't put

           @foo{$a,$b,$c}      # a slice--note the @

          which means

           ($foo{$a},$foo{$b},$foo{$c})

          Default is "\034", the same as SUBSEP in awk.  Note that if
          your keys contain binary data there might not be any safe
          value for $;.  (Mnemonic: comma (the syntactic subscript
          separator) is a semi-semicolon.  Yeah, I know, it's pretty
          lame, but $, is already taken for something more important.)

      $!      If used in a numeric context, yields the current value of
          errno, with all the usual caveats.  (This means that you
          shouldn't depend on the value of $! to be anything in
          particular unless you've gotten a specific error return
          indicating a system error.) If used in a string context,
          yields the corresponding system error string.  You can assign
          to $! in order to set errno if, for instance, you want $! to
          return the string for error n, or you want to set the exit
          value for the die operator.  (Mnemonic: What just went bang?)

      $@      The perl syntax error message from the last eval command.  If
          null, the last eval parsed and executed correctly (although
          the operations you invoked may have failed in the normal
          fashion).  (Mnemonic: Where was the syntax error "at"?)

      $<      The real uid of this process.  (Mnemonic: it's the uid you
          came FROM, if you're running setuid.)

      $>      The effective uid of this process.  Example:

           $< = $>;  # set real uid to the effective uid
           ($<,$>) = ($>,$<);  # swap real and effective uid

          (Mnemonic: it's the uid you went TO, if you're running
          setuid.) Note: $< and $> can only be swapped on machines
          supporting setreuid().

      $(      The real gid of this process.  If you are on a machine that
          supports membership in multiple groups simultaneously, gives a
          space separated list of groups you are in.  The first number
          is the one returned by getgid(), and the subsequent ones by
          getgroups(), one of which may be the same as the first number.
          (Mnemonic: parentheses are used to GROUP things.    The real gid
          is the group you LEFT, if you're running setgid.)

      $)      The effective gid of this process.  If you are on a machine
          that supports membership in multiple groups simultaneously,
          gives a space separated list of groups you are in.  The first
          number is the one returned by getegid(), and the subsequent
          ones by getgroups(), one of which may be the same as the first
          number.  (Mnemonic: parentheses are used to GROUP things.  The
          effective gid is the group that's RIGHT for you, if you're
          running setgid.)

          Note: $<, $>, $( and $) can only be set on machines that
          support the corresponding set[re][ug]id() routine.  $( and $)
          can only be swapped on machines supporting setregid().

      $:      The current set of characters after which a string may be
          broken to fill continuation fields (starting with ^) in a
          format.  Default is " \n-", to break on whitespace or hyphens.
          (Mnemonic: a "colon" in poetry is a part of a line.)

      $^D     The current value of the debugging flags.  (Mnemonic: value of
          -D switch.)

      $^F     The maximum system file descriptor, ordinarily 2.  System file
          descriptors are passed to subprocesses, while higher file
          descriptors are not.  During an open, system file descriptors
          are preserved even if the open fails.  Ordinary file
          descriptors are closed before the open is attempted.

      $^I     The current value of the inplace-edit extension.    Use undef to
          disable inplace editing.    (Mnemonic: value of -i switch.)

      $^L     What formats output to perform a formfeed.  Default is \f.

      $^P     The internal flag that the debugger clears so that it doesn't
          debug itself.  You could conceivable disable debugging
          yourself by clearing it.

      $^T     The time at which the script began running, in seconds since
          the epoch.  The values returned by the -M , -A and -C
          filetests are based on this value.

      $^W     The current value of the warning switch.    (Mnemonic: related
          to the -w switch.)

      $^X     The name that Perl itself was executed as, from argv[0].

      $ARGV   contains the name of the current file when reading from <>.

      @ARGV   The array ARGV contains the command line arguments intended
          for the script.  Note that $#ARGV is the generally number of
          arguments minus one, since $ARGV[0] is the first argument, NOT
          the command name.  See $0 for the command name.

      @INC    The array INC contains the list of places to look for perl
          scripts to be evaluated by the "do EXPR" command or the
          "require" command.  It initially consists of the arguments to
          any -I command line switches, followed by the default perl
          library, probably "/usr/local/lib/perl", followed by ".", to
          represent the current directory.

      %INC    The associative array INC contains entries for each filename
          that has been included via "do" or "require".  The key is the
          filename you specified, and the value is the location of the
          file actually found.  The "require" command uses this array to
          determine whether a given file has already been included.

      $ENV{expr}
          The associative array ENV contains your current environment.
          Setting a value in ENV changes the environment for child
          processes.

      $SIG{expr}
          The associative array SIG is used to set signal handlers for
          various signals.    Example:

           sub handler {  # 1st argument is signal name
            local($sig) = @_;
            print "Caught a SIG$sig--shutting down\n";
            close(LOG);
            exit(0);
           }

           $SIG{'INT'} = 'handler';
           $SIG{'QUIT'} = 'handler';
           ...
           $SIG{'INT'} = 'DEFAULT'; # restore default action
           $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT

          The SIG array only contains values for the signals actually
          set within the perl script.

      Packages

      Perl provides a mechanism for alternate namespaces to protect packages
      from stomping on each others variables.  By default, a perl script
      starts compiling into the package known as "main".  By use of the
      package declaration, you can switch namespaces.  The scope of the
      package declaration is from the declaration itself to the end of the
      enclosing block (the same scope as the local() operator).  Typically
      it would be the first declaration in a file to be included by the
      "require" operator.  You can switch into a package in more than one
      place; it merely influences which symbol table is used by the compiler
      for the rest of that block.  You can refer to variables and
      filehandles in other packages by prefixing the identifier with the
      package name and a single quote.    If the package name is null, the
      "main" package as assumed.

      Only identifiers starting with letters are stored in the packages
      symbol table.  All other symbols are kept in package "main".  In
      addition, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV,
      INC and SIG are forced to be in package "main", even when used for
      other purposes than their built-in one.  Note also that, if you have a
      package called "m", "s" or "y", the you can't use the qualified form
      of an identifier since it will be interpreted instead as a pattern
      match, a substitution or a translation.

      Eval'ed strings are compiled in the package in which the eval was
      compiled in.  (Assignments to $SIG{}, however, assume the signal
      handler specified is in the main package.  Qualify the signal handler
      name if you wish to have a signal handler in a package.) For an
      example, examine perldb.pl in the perl library.  It initially switches
      to the DB package so that the debugger doesn't interfere with
      variables in the script you are trying to debug.    At various points,
      however, it temporarily switches back to the main package to evaluate
      various expressions in the context of the main package.

      The symbol table for a package happens to be stored in the associative
      array of that name prepended with an underscore.    The value in each
      entry of the associative array is what you are referring to when you
      use the *name notation.  In fact, the following have the same effect
      (in package main, anyway), though the first is more efficient because
      it does the symbol table lookups at compile time:

       local(*foo) = *bar;
       local($_main{'foo'}) = $_main{'bar'};

      You can use this to print out all the variables in a package, for
      instance.  Here is dumpvar.pl from the perl library:
       package dumpvar;

       sub main'dumpvar {
           ($package) = @_;
           local(*stab) = eval("*_$package");
           while (($key,$val) = each(%stab)) {
           {
               local(*entry) = $val;
               if (defined $entry) {
               print "\$$key = '$entry'\n";
               }
               if (defined @entry) {
               print "\@$key = (\n";
               foreach $num ($[ .. $#entry) {
                   print "    $num\t'",$entry[$num],"'\n";
               }
               print ")\n";
               }
               if ($key ne "_$package" && defined %entry) {
               print "\%$key = (\n";
               foreach $key (sort keys(%entry)) {
                   print "    $key\t'",$entry{$key},"'\n";
               }
               print ")\n";
               }
           }
           }
       }

      Note that, even though the subroutine is compiled in package dumpvar,
      the name of the subroutine is qualified so that its name is inserted
      into package "main".

      Style

      Each programmer will, of course, have his or her own preferences in
      regards to formatting, but there are some general guidelines that will
      make your programs easier to read.

      1.  Just because you CAN do something a particular way doesn't mean
      that you SHOULD do it that way.  Perl is designed to give you
      several ways to do anything, so consider picking the most readable
      one.    For instance

           open(FOO,$foo) || die "Can't open $foo: $!";

      is better than

           die "Can't open $foo: $!" unless open(FOO,$foo);

      because the second way hides the main point of the statement in a
      modifier.  On the other hand

           print "Starting analysis\n" if $verbose;

      is better than

           $verbose && print "Starting analysis\n";

      since the main point isn't whether the user typed -v or not.

      Similarly, just because an operator lets you assume default
      arguments doesn't mean that you have to make use of the defaults.
      The defaults are there for lazy systems programmers writing one-
      shot programs.  If you want your program to be readable, consider
      supplying the argument.

      Along the same lines, just because you can omit parentheses in
      many places doesn't mean that you ought to:

           return print reverse sort num values array;
           return print(reverse(sort num (values(%array))));

      When in doubt, parenthesize.    At the very least it will let some
      poor schmuck bounce on the % key in vi.

      Even if you aren't in doubt, consider the mental welfare of the
      person who has to maintain the code after you, and who will
      probably put parens in the wrong place.

      2.  Don't go through silly contortions to exit a loop at the top or
      the bottom, when perl provides the "last" operator so you can exit
      in the middle.  Just outdent it a little to make it more visible:

          line:
           for (;;) {
           statements;
           last line if $foo;
           next line if /^#/;
           statements;
           }


      3.  Don't be afraid to use loop labels--they're there to enhance
      readability as well as to allow multi-level loop breaks.  See last
      example.

      4.  For portability, when using features that may not be implemented
      on every machine, test the construct in an eval to see if it
      fails.  If you know what version or patchlevel a particular
      feature was implemented, you can test $] to see if it will be
      there.

      5.  Choose mnemonic identifiers.

      6.  Be consistent.

      Debugging

      If you invoke perl with a -d switch, your script will be run under a
      debugging monitor.  It will halt before the first executable statement
      and ask you for a command, such as:

      h       Prints out a help message.

      T       Stack trace.

      s       Single step.    Executes until it reaches the beginning of
          another statement.

      n       Next.  Executes over subroutine calls, until it reaches
          the beginning of the next statement.

      f       Finish.  Executes statements until it has finished the
          current subroutine.

      c       Continue.  Executes until the next breakpoint is reached.

      c line      Continue to the specified line.  Inserts a one-time-only
          breakpoint at the specified line.

      <CR>      Repeat last n or s.

      l min+incr  List incr+1 lines starting at min.  If min is omitted,
          starts where last listing left off.  If incr is omitted,
          previous value of incr is used.

      l min-max   List lines in the indicated range.

      l line      List just the indicated line.

      l       List next window.

      -       List previous window.

      w line      List window around line.

      l subname   List subroutine.  If it's a long subroutine it just lists
          the beginning.  Use "l" to list more.

      /pattern/   Regular expression search forward for pattern; the final /
          is optional.

      ?pattern?   Regular expression search backward for pattern; the final
          ? is optional.

      L       List lines that have breakpoints or actions.

      S       Lists the names of all subroutines.

      t       Toggle trace mode on or off.

      b line condition
          Set a breakpoint.  If line is omitted, sets a breakpoint
          on the line that is about to be executed.  If a condition
          is specified, it is evaluated each time the statement is
          reached and a breakpoint is taken only if the condition is
          true.  Breakpoints may only be set on lines that begin an
          executable statement.

      b subname condition
          Set breakpoint at first executable line of subroutine.

      d line      Delete breakpoint.  If line is omitted, deletes the
          breakpoint on the line that is about to be executed.

      D       Delete all breakpoints.

      a line command
          Set an action for line.  A multi-line command may be
          entered by backslashing the newlines.

      A       Delete all line actions.

      < command   Set an action to happen before every debugger prompt.  A
          multi-line command may be entered by backslashing the
          newlines.

      > command   Set an action to happen after the prompt when you've just
          given a command to return to executing the script.  A
          multi-line command may be entered by backslashing the
          newlines.

      V package   List all variables in package.  Default is main package.

      ! number      Redo a debugging command.  If number is omitted, redoes
          the previous command.

      ! -number   Redo the command that was that many commands ago.

      H -number   Display last n commands.  Only commands longer than one
          character are listed.  If number is omitted, lists them
          all.

      q or ^D      Quit.

      command      Execute command as a perl statement.    A missing semicolon
          will be supplied.

      p expr      Same as "print DB'OUT expr".    The DB'OUT filehandle is
          opened to /dev/tty, regardless of where STDOUT may be
          redirected to.

      If you want to modify the debugger, copy perldb.pl from the perl
      library to your current directory and modify it as necessary.  (You'll
      also have to put -I. on your command line.) You can do some
      customization by setting up a .perldb file which contains
      initialization code.  For instance, you could make aliases like these:

      $DB'alias{'len'} = 's/^len(.*)/p length($1)/';
      $DB'alias{'stop'} = 's/^stop (at|in)/b/';
      $DB'alias{'.'} =
        's/^\./p "\$DB\'sub(\$DB\'line):\t",\$DB\'line[\$DB\'line]/';


      Setuid Scripts

      Perl is designed to make it easy to write secure setuid and setgid
      scripts.    Unlike shells, which are based on multiple substitution
      passes on each line of the script, perl uses a more conventional
      evaluation scheme with fewer hidden "gotchas".  Additionally, since
      the language has more built-in functionality, it has to rely less upon
      external (and possibly untrustworthy) programs to accomplish its
      purposes.

      In an unpatched 4.2 or 4.3bsd kernel, setuid scripts are intrinsically
      insecure, but this kernel feature can be disabled.  If it is, perl can
      emulate the setuid and setgid mechanism when it notices the otherwise
      useless setuid/gid bits on perl scripts.    If the kernel feature isn't
      disabled, perl will complain loudly that your setuid script is
      insecure.  You'll need to either disable the kernel setuid script
      feature, or put a C wrapper around the script.

      When perl is executing a setuid script, it takes special precautions
      to prevent you from falling into any obvious traps.  (In some ways, a
      perl script is more secure than the corresponding C program.) Any
      command line argument, environment variable, or input is marked as
      "tainted", and may not be used, directly or indirectly, in any command
      that invokes a subshell, or in any command that modifies files,
      directories or processes.  Any variable that is set within an
      expression that has previously referenced a tainted value also becomes
      tainted (even if it is logically impossible for the tainted value to
      influence the variable).    For example:

       $foo = shift;        # $foo is tainted
       $bar = $foo,'bar';        # $bar is also tainted
       $xxx = <>;            # Tainted
       $path = $ENV{'PATH'};    # Tainted, but see below
       $abc = 'abc';        # Not tainted

       system "echo $foo";        # Insecure
       system "/bin/echo", $foo;     # Secure (doesn't use sh)
       system "echo $bar";        # Insecure
       system "echo $abc";        # Insecure until PATH set

       $ENV{'PATH'} = '/bin:/usr/bin';
       $ENV{'IFS'} = '' if $ENV{'IFS'} ne '';

       $path = $ENV{'PATH'};    # Not tainted
       system "echo $abc";        # Is secure now!

       open(FOO,"$foo");        # OK
       open(FOO,">$foo");        # Not OK

       open(FOO,"echo $foo|");  # Not OK, but...
       open(FOO,"-|") || exec 'echo', $foo;    # OK

       $zzz = `echo $foo`;        # Insecure, zzz tainted

       unlink $abc,$foo;        # Insecure
       umask $foo;            # Insecure

       exec "echo $foo";        # Insecure
       exec "echo", $foo;        # Secure (doesn't use sh)
       exec "sh", '-c', $foo;   # Considered secure, alas

      The taintedness is associated with each scalar value, so some elements
      of an array can be tainted, and others not.

      If you try to do something insecure, you will get a fatal error saying
      something like "Insecure dependency" or "Insecure PATH".    Note that
      you can still write an insecure system call or exec, but only by
      explicitly doing something like the last example above.  You can also
      bypass the tainting mechanism by referencing subpatterns--perl
      presumes that if you reference a substring using $1, $2, etc, you knew
      what you were doing when you wrote the pattern:

       $ARGV[0] =~ /^-P(\w+)$/;
       $printer = $1;      # Not tainted

      This is fairly secure since \w+ doesn't match shell metacharacters.
      Use of .+ would have been insecure, but perl doesn't check for that,
      so you must be careful with your patterns.  This is the ONLY mechanism
      for untainting user supplied filenames if you want to do file
      operations on them (unless you make $> equal to $<).

      It's also possible to get into trouble with other operations that
      don't care whether they use tainted values.  Make judicious use of the
      file tests in dealing with any user-supplied filenames.  When
      possible, do opens and such after setting $> = $<.  Perl doesn't
      prevent you from opening tainted filenames for reading, so be careful
      what you print out.  The tainting mechanism is intended to prevent
      stupid mistakes, not to remove the need for thought.

 ENVIRONMENT
      HOME      Used if chdir has no argument.

      LOGDIR      Used if chdir has no argument and HOME is not set.

      PATH      Used in executing subprocesses, and in finding the script
          if -S is used.

      PERLLIB      A colon-separated list of directories in which to look for
          Perl library files before looking in the standard library
          and the current directory.

      PERLDB      The command used to get the debugger code.  If unset, uses

               require 'perldb.pl'


      Apart from these, perl uses no other environment variables, except to
      make them available to the script being executed, and to child
      processes.  However, scripts running setuid would do well to execute
      the following lines before doing anything else, just to keep people
      honest:

      $ENV{'PATH'} = '/bin:/usr/bin';    # or whatever you need
      $ENV{'SHELL'} = '/bin/sh' if $ENV{'SHELL'} ne '';
      $ENV{'IFS'} = '' if $ENV{'IFS'} ne '';


 AUTHOR
      Larry Wall <[e-mail]>
      MS-DOS port by Diomidis Spinellis <[e-mail]>

 FILES
      /tmp/perl-eXXXXXX   temporary file for -e commands.

 SEE ALSO
      a2p  awk to perl translator
      s2p  sed to perl translator

 DIAGNOSTICS
      Compilation errors will tell you the line number of the error, with an
      indication of the next token or token type that was to be examined.
      (In the case of a script passed to perl via -e switches, each -e is
      counted as one line.)

      Setuid scripts have additional constraints that can produce error
      messages such as "Insecure dependency".  See the section on setuid
      scripts.

 TRAPS
      Accustomed awk users should take special note of the following:

      *   Semicolons are required after all simple statements in perl
      (except at the end of a block).  Newline is not a statement
      delimiter.

      *   Curly brackets are required on ifs and whiles.

      *   Variables begin with $ or @ in perl.

      *   Arrays index from 0 unless you set $[.  Likewise string positions
      in substr() and index().

      *   You have to decide whether your array has numeric or string
      indices.

      *   Associative array values do not spring into existence upon mere
      reference.

      *   You have to decide whether you want to use string or numeric
      comparisons.

      *   Reading an input line does not split it for you.  You get to split
      it yourself to an array.  And the split operator has different
      arguments.

      *   The current input line is normally in $_, not $0.  It generally
      does not have the newline stripped.  ($0 is the name of the
      program executed.)

      *   $<digit> does not refer to fields--it refers to substrings matched
      by the last match pattern.

      *   The print statement does not add field and record separators
      unless you set $, and $\.

      *   You must open your files before you print to them.

      *   The range operator is "..", not comma.  (The comma operator works
      as in C.)

      *   The match operator is "=~", not "~".    ("~" is the one's complement
      operator, as in C.)

      *   The exponentiation operator is "**", not "^".  ("^" is the XOR
      operator, as in C.)

      *   The concatenation operator is ".", not the null string.  (Using
      the null string would render "/pat/ /pat/" unparsable, since the
      third slash would be interpreted as a division operator--the
      tokener is in fact slightly context sensitive for operators like
      /, ?, and <.    And in fact, . itself can be the beginning of a
      number.)

      *   Next, exit and continue work differently.

      *   The following variables work differently

         Awk           Perl
         ARGC           $#ARGV
         ARGV[0]       $0
         FILENAME       $ARGV
         FNR           $. - something
         FS           (whatever you like)
         NF           $#Fld, or some such
         NR           $.
         OFMT           $#
         OFS           $,
         ORS           $\
         RLENGTH       length($&)
         RS           $/
         RSTART        length($`)
         SUBSEP        $;


      *   When in doubt, run the awk construct through a2p and see what it
      gives you.

      Cerebral C programmers should take note of the following:

      *   Curly brackets are required on ifs and whiles.

      *   You should use "elsif" rather than "else if"

      *   Break and continue become last and next, respectively.

      *   There's no switch statement.

      *   Variables begin with $ or @ in perl.

      *   Printf does not implement *.

      *   Comments begin with #, not /*.

      *   You can't take the address of anything.

      *   ARGV must be capitalized.

      *   The "system" calls link, unlink, rename, etc. return nonzero for
      success, not 0.

      *   Signal handlers deal with signal names, not numbers.

      Seasoned sed programmers should take note of the following:

      *   Backreferences in substitutions use $ rather than \.

      *   The pattern matching metacharacters (, ), and | do not have
      backslashes in front.

      *   The range operator is .. rather than comma.

      Sharp shell programmers should take note of the following:

      *   The backtick operator does variable interpretation without regard
      to the presence of single quotes in the command.

      *   The backtick operator does no translation of the return value,
      unlike csh.

      *   Shells (especially csh) do several levels of substitution on each
      command line.  Perl does substitution only in certain constructs
      such as double quotes, backticks, angle brackets and search
      patterns.

      *   Shells interpret scripts a little bit at a time.  Perl compiles
      the whole program before executing it.

      *   The arguments are available via @ARGV, not $1, $2, etc.

      *   The environment is not automatically made available as variables.

 ERRATA AND ADDENDA
      The Perl book, Programming Perl , has the following omissions and
      goofs.

      On page 5, the examples which read

       eval "/usr/bin/perl

      should read

       eval "exec /usr/bin/perl


      On page 195, the equivalent to the System V sum program only works for
      very small files.  To do larger files, use

       undef $/;
       $checksum = unpack("%32C*",<>) % 32767;


      The descriptions of alarm and sleep refer to signal SIGALARM.  These
      should refer to SIGALRM.

      The -0 switch to set the initial value of $/ was added to Perl after
      the book went to press.

      The -l switch now does automatic line ending processing.

      The qx// construct is now a synonym for backticks.

      $0 may now be assigned to set the argument displayed by ps (1).

      The new @###.## format was omitted accidentally from the description
      on formats.

      It wasn't known at press time that s///ee caused multiple evaluations
      of the replacement expression.  This is to be construed as a feature.

      (LIST) x $count now does array replication.

      There is now no limit on the number of parentheses in a regular
      expression.

      In double-quote context, more escapes are supported: \e, \a, \x1b,
      \c[, \l, \L, \u, \U, \E.    The latter five control up/lower case
      translation.

      The $/ variable may now be set to a multi-character delimiter.

      There is now a g modifier on ordinary pattern matching that causes it
      to iterate through a string finding multiple matches.

      All of the $^X variables are new except for $^T.

      The default top-of-form format for FILEHANDLE is now FILEHANDLE_TOP
      rather than top.

      The eval {} and sort {} constructs were added in version 4.018.

      The v and V (little-endian) template options for pack and unpack were
      added in 4.019.

 BUGS
      Perl is at the mercy of your machine's definitions of various
      operations such as type casting, atof() and sprintf().

      If your stdio requires an seek or eof between reads and writes on a
      particular stream, so does perl.    (This doesn't apply to sysread() and
      syswrite().)

      While none of the built-in data types have any arbitrary size limits
      (apart from memory size), there are still a few arbitrary limits: a
      given identifier may not be longer than 255 characters, and no
      component of your PATH may be longer than 255 if you use -S.  A
      regular expression may not compile to more than 32767 bytes
      internally.

      Perl actually stands for Pathologically Eclectic Rubbish Lister, but
      don't tell anyone I said that.