Acorn User 10/AU_CD10.iso/Updates/Perl/Non-RPC/Docs/perlvar

Text File | 1999-04-17 | 39KB | 983 lines

NAME
    perlvar - Perl predefined variables

DESCRIPTION
  Predefined Names

    The following names have special meaning to Perl. Most
    punctuation names have reasonable mnemonics, or analogues in one
    of the shells. Nevertheless, if you wish to use long variable
    names, you just need to say

        use English;


    at the top of your program. This will alias all the short names
    to the long names in the current package. Some even have medium
    names, generally borrowed from awk.

    Due to an unfortunate accident of Perl's implementation, "`use
    English'" imposes a considerable performance penalty on all
    regular expression matches in a program, regardless of whether
    they occur in the scope of "`use English'". For that reason,
    saying "`use English'" in libraries is strongly discouraged. See
    the Devel::SawAmpersand module documentation from CPAN
    (http://www.perl.com/CPAN/modules/by-module/Devel/Devel-
    SawAmpersand-0.10.readme) for more information.

    To go a step further, those variables that depend on the
    currently selected filehandle may instead (and preferably) be
    set by calling an object method on the FileHandle object.
    (Summary lines below for this contain the word HANDLE.) First
    you must say

        use FileHandle;


    after which you may use either

        method HANDLE EXPR


    or more safely,

        HANDLE->method(EXPR)


    Each of the methods returns the old value of the FileHandle
    attribute. The methods each take an optional EXPR, which if
    supplied specifies the new value for the FileHandle attribute in
    question. If not supplied, most of the methods do nothing to the
    current value, except for autoflush(), which will assume a 1 for
    you, just to be different.

    A few of these variables are considered "read-only". This means
    that if you try to assign to this variable, either directly or
    indirectly through a reference, you'll raise a run-time
    exception.

    The following list is ordered by scalar variables first, then
    the arrays, then the hashes (except $^M was added in the wrong
    place). This is somewhat obscured by the fact that %ENV and %SIG
    are listed as $ENV{expr} and $SIG{expr}.

    $ARG

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

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

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

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

                chop
                chop($_)


            Here are the places where Perl will assume $_ even if
            you don't use it:

    *          Various unary functions, including functions like ord()
               and int(), as well as the all file tests (`-f', `-d')
               except for `-t', which defaults to STDIN.

    *          Various list functions like print() and unlink().

    *          The pattern matching operations `m//', `s///', and
               `tr///' when used without an `=~' operator.

    *          The default iterator variable in a `foreach' loop if no
               other variable is supplied.

    *          The implicit iterator variable in the grep() and map()
               functions.

    *          The default place to put an input record when a `<FH>'
               operation's result is tested by itself as the sole
               criterion of a `while' test. Note that outside of a
               `while' test, this will not happen.


            (Mnemonic: underline is understood in certain
            operations.)


    $<*digits*>
            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 \digits.) These variables are
            all read-only.

    $MATCH

    $&      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.) This variable is read-only.

            The use of this variable anywhere in a program imposes a
            considerable performance penalty on all regular
            expression matches. See the Devel::SawAmpersand module
            from CPAN for more information.

    $PREMATCH

    $`      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.)
            This variable is read-only.

            The use of this variable anywhere in a program imposes a
            considerable performance penalty on all regular
            expression matches. See the Devel::SawAmpersand module
            from CPAN for more information.

    $POSTMATCH

    $'      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


            This variable is read-only.

            The use of this variable anywhere in a program imposes a
            considerable performance penalty on all regular
            expression matches. See the Devel::SawAmpersand module
            from CPAN for more information.

    $LAST_PAREN_MATCH

    $+      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.) This
            variable is read-only.

    $MULTILINE_MATCHING

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

            Use of "`$*'" is deprecated in modern Perls, supplanted
            by the `/s' and `/m' modifiers on pattern matching.

    input_line_number HANDLE EXPR

    $INPUT_LINE_NUMBER

    $NR

    $.      The current input line number for the last file handle from
            which you read (or performed a `seek' or `tell' on). The
            value may be different from the actual physical line
            number in the file, depending on what notion of "line"
            is in effect--see the $ manpage on how to affect that.
            An explicit close on a filehandle resets the line
            number. Because "`<>'" never does an explicit close,
            line numbers increase across ARGV files (but see
            examples under eof()). Localizing `$.' has the effect of
            also localizing Perl's notion of "the last read
            filehandle". (Mnemonic: many programs use "." to mean
            the current line number.)

    input_record_separator HANDLE EXPR

    $INPUT_RECORD_SEPARATOR

    $RS

    $/      The input record separator, newline by default. This is used
            to influence Perl's idea of what a "line" is. Works like
            awk's RS variable, including treating empty lines as
            delimiters if set to the null string. (Note: An empty
            line cannot contain any spaces or tabs.) You may set it
            to a multi-character string to match a multi-character
            delimiter, or to `undef' to read to end of file. Note
            that setting it to `"\n\n"' means something slightly
            different than setting it to `""', if the file contains
            consecutive empty lines. Setting it to `""' will treat
            two or more consecutive empty lines as a single empty
            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.)

                undef $/;        # enable "slurp" mode
                $_ = <FH>;        # whole file now here
                s/\n[ \t]+/ /g;


            Remember: the value of $/ is a string, not a regexp. AWK
            has to be better for something :-)

            Setting $/ to a reference to an integer, scalar
            containing an integer, or scalar that's convertable to
            an integer will attempt to read records instead of
            lines, with the maximum record size being the referenced
            integer. So this:

                $/ = \32768; # or \"32768", or \$var_containing_32768
                open(FILE, $myfile);
                $_ = <FILE>;


            will read a record of no more than 32768 bytes from
            FILE. If you're not reading from a record-oriented file
            (or your OS doesn't have record-oriented files), then
            you'll likely get a full chunk of data with every read.
            If a record is larger than the record size you've set,
            you'll get the record back in pieces.

            On VMS, record reads are done with the equivalent of
            `sysread', so it's best not to mix record and non-record
            reads on the same file. (This is likely not a problem,
            as any file you'd want to read in record mode is
            probably usable in line mode) Non-VMS systems perform
            normal I/O, so it's safe to mix record and non-record
            reads of a file.

            Also see the $. manpage.

    autoflush HANDLE EXPR

    $OUTPUT_AUTOFLUSH

    $|      If set to nonzero, forces a flush right away and after every
            write or print on the currently selected output channel.
            Default is 0 (regardless of whether the channel is
            actually buffered by the system or not; `$|' tells you
            only whether you've asked Perl explicitly to flush after
            each write). 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. This has no effect on input
            buffering. (Mnemonic: when you want your pipes to be
            piping hot.)

    output_field_separator HANDLE EXPR

    $OUTPUT_FIELD_SEPARATOR

    $OFS

    $,      The output field separator for the print operator.
            Ordinarily the print operator simply prints out the
            comma-separated fields you specify. 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.)

    output_record_separator HANDLE EXPR

    $OUTPUT_RECORD_SEPARATOR

    $ORS

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

    $LIST_SEPARATOR

    $"      This is like "`$,'" 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.)

    $SUBSCRIPT_SEPARATOR

    $SUBSEP

    $;      The subscript separator for multidimensional array
            emulation. If you refer to a hash 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.)

            Consider using "real" multidimensional arrays.

    $OFMT

    $#      The output format for printed numbers. This variable is a
            half-hearted attempt to emulate awk's OFMT variable.
            There are times, however, when awk and Perl have
            differing notions of what is in fact numeric. The
            initial value is %.*n*g, where *n* is the value of the
            macro DBL_DIG from your system's float.h. This is
            different from awk's default OFMT setting of %.6g, so
            you need to set "`$#'" explicitly to get awk's value.
            (Mnemonic: # is the number sign.)

            Use of "`$#'" is deprecated.

    format_page_number HANDLE EXPR

    $FORMAT_PAGE_NUMBER

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

    format_lines_per_page HANDLE EXPR

    $FORMAT_LINES_PER_PAGE

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

    format_lines_left HANDLE EXPR

    $FORMAT_LINES_LEFT

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

    format_name HANDLE EXPR

    $FORMAT_NAME

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

    format_top_name HANDLE EXPR

    $FORMAT_TOP_NAME

    $^      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.)

    format_line_break_characters HANDLE EXPR

    $FORMAT_LINE_BREAK_CHARACTERS

    $:      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.)

    format_formfeed HANDLE EXPR

    $FORMAT_FORMFEED

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

    $ACCUMULATOR

    $^A     The current value of the write() accumulator for format()
            lines. A format contains formline() commands that put
            their result into `$^A'. After calling its format,
            write() prints out the contents of `$^A' and empties. So
            you never actually see the contents of `$^A' unless you
            call formline() yourself and then look at it. See the
            perlform manpage and the "formline()" entry in the
            perlfunc manpage.

    $CHILD_ERROR

    $?      The status returned by the last pipe close, backtick (```')
            command, or system() operator. Note that this is the
            status word returned by the wait() system call (or else
            is made up to look like it). Thus, the exit value of the
            subprocess is actually (`$? >> 8'), and `$? & 127' gives
            which signal, if any, the process died from, and `$? &
            128' reports whether there was a core dump. (Mnemonic:
            similar to sh and ksh.)

            Additionally, if the `h_errno' variable is supported in
            C, its value is returned via $? if any of the
            `gethost*()' functions fail.

            Note that if you have installed a signal handler for
            `SIGCHLD', the value of `$?' will usually be wrong
            outside that handler.

            Inside an `END' subroutine `$?' contains the value that
            is going to be given to `exit()'. You can modify `$?' in
            an `END' subroutine to change the exit status of the
            script.

            Under VMS, the pragma `use vmsish 'status'' makes `$?'
            reflect the actual VMS exit status, instead of the
            default emulation of POSIX status.

            Also see the Error Indicators manpage.

    $OS_ERROR

    $ERRNO

    $!      If used in a numeric context, yields the current value of
            errno, with all the usual caveats. (This means that you
            shouldn't depend on the value of `$!' 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 `$!' to set *errno* if, for instance, you want
            `"$!"' to return the string for error *n*, or you want
            to set the exit value for the die() operator. (Mnemonic:
            What just went bang?)

            Also see the Error Indicators manpage.

    $EXTENDED_OS_ERROR

    $^E     Error information specific to the current operating system.
            At the moment, this differs from `$!' under only VMS,
            OS/2, and Win32 (and for MacPerl). On all other
            platforms, `$^E' is always just the same as `$!'.

            Under VMS, `$^E' provides the VMS status value from the
            last system error. This is more specific information
            about the last system error than that provided by `$!'.
            This is particularly important when `$!' is set to
            EVMSERR.

            Under OS/2, `$^E' is set to the error code of the last
            call to OS/2 API either via CRT, or directly from perl.

            Under Win32, `$^E' always returns the last error
            information reported by the Win32 call `GetLastError()'
            which describes the last error from within the Win32
            API. Most Win32-specific code will report errors via
            `$^E'. ANSI C and UNIX-like calls set `errno' and so
            most portable Perl code will report errors via `$!'.

            Caveats mentioned in the description of `$!' generally
            apply to `$^E', also. (Mnemonic: Extra error
            explanation.)

            Also see the Error Indicators manpage.

    $EVAL_ERROR

    $@      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"?)

            Note that warning messages are not collected in this
            variable. You can, however, set up a routine to process
            warnings by setting `$SIG{__WARN__}' as described below.

            Also see the Error Indicators manpage.

    $PROCESS_ID

    $PID

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

    $REAL_USER_ID

    $UID

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

    $EFFECTIVE_USER_ID

    $EUID

    $>      The effective uid of this process. Example:

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


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

    $REAL_GROUP_ID

    $GID

    $(      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.

            However, a value assigned to "`$('" must be a single
            number used to set the real gid. So the value given by
            "`$('" should *not* be assigned back to "`$('" without
            being forced numeric, such as by adding zero.

            (Mnemonic: parentheses are used to *GROUP* things. The
            real gid is the group you *LEFT*, if you're running
            setgid.)

    $EFFECTIVE_GROUP_ID

    $EGID

    $)      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.

            Similarly, a value assigned to "`$)'" must also be a
            space-separated list of numbers. The first number is
            used to set the effective gid, and the rest (if any) are
            passed to setgroups(). To get the effect of an empty
            list for setgroups(), just repeat the new effective gid;
            that is, to force an effective gid of 5 and an
            effectively empty setgroups() list, say ` $) = "5 5" '.

            (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 be set only
            on machines that support the corresponding
            *set[re][ug]id()* routine. "`$('" and "`$)'" can be
            swapped only on machines supporting setregid().

    $PROGRAM_NAME

    $0      Contains the name of the file containing the Perl script
            being executed. On some operating systems assigning to
            "`$0'" modifies the argument area that the ps(1) program
            sees. This is more useful as a way of indicating the
            current program state than it is for hiding the program
            you're running. (Mnemonic: same as sh and ksh.)

    $[      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.)

            As of Perl 5, assignment to "`$['" is treated as a
            compiler directive, and cannot influence the behavior of
            any other file. Its use is discouraged.

    $PERL_VERSION

    $]      The version + patchlevel / 1000 of the Perl interpreter.
            This variable can be used to determine whether the Perl
            interpreter executing a script is in the right range of
            versions. (Mnemonic: Is this version of perl in the
            right bracket?) Example:

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


            See also the documentation of `use VERSION' and `require
            VERSION' for a convenient way to fail if the Perl
            interpreter is too old.

    $COMPILING

    $^C     The current value of the flag associated with the -c switch.
            Mainly of use with -MO=... to allow code to alter its
            behaviour when being compiled. (For example to
            automatically AUTOLOADing at compile time rather than
            normal deferred loading.) Setting `$^C = 1' is similar
            to calling `B::minus_c'.

    $DEBUGGING

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

    $SYSTEM_FD_MAX

    $^F     The maximum system file descriptor, ordinarily 2. System
            file descriptors are passed to exec()ed processes, while
            higher file descriptors are not. Also, during an open(),
            system file descriptors are preserved even if the open()
            fails. (Ordinary file descriptors are closed before the
            open() is attempted.) Note that the close-on-exec status
            of a file descriptor will be decided according to the
            value of `$^F' when the open() or pipe() was called, not
            the time of the exec().

    $^H     The current set of syntax checks enabled by `use strict' and
            other block scoped compiler hints. See the documentation
            of `strict' for more details.

    $INPLACE_EDIT

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

    $^M     By default, running out of memory it is not trappable.
            However, if compiled for this, Perl may use the contents
            of `$^M' as an emergency pool after die()ing with this
            message. Suppose that your Perl were compiled with -
            DPERL_EMERGENCY_SBRK and used Perl's malloc. Then

                $^M = 'a' x (1<<16);


            would allocate a 64K buffer for use when in emergency.
            See the INSTALL file for information on how to enable
            this option. As a disincentive to casual use of this
            advanced feature, there is no the English manpage long
            name for this variable.

    $OSNAME

    $^O     The name of the operating system under which this copy of
            Perl was built, as determined during the configuration
            process. The value is identical to `$Config{'osname'}'.

    $PERLDB

    $^P     The internal variable for debugging support. Different bits
            mean the following (subject to change):

    0x01          Debug subroutine enter/exit.

    0x02          Line-by-line debugging.

    0x04          Switch off optimizations.

    0x08          Preserve more data for future interactive inspections.

    0x10          Keep info about source lines on which a subroutine is
                  defined.

    0x20          Start with single-step on.


            Note that some bits may be relevant at compile-time
            only, some at run-time only. This is a new mechanism and
            the details may change.

    $^R     The result of evaluation of the last successful the "`(?{
            code })'" entry in the perlre manpage regular expression
            assertion. (Excluding those used as switches.) May be
            written to.

    $^S     Current state of the interpreter. Undefined if parsing of
            the current module/eval is not finished (may happen in
            $SIG{__DIE__} and $SIG{__WARN__} handlers). True if
            inside an eval, otherwise false.

    $BASETIME

    $^T     The time at which the script began running, in seconds since
            the epoch (beginning of 1970). The values returned by
            the -M, -A, and -C filetests are based on this value.

    $WARNING

    $^W     The current value of the warning switch, either TRUE or
            FALSE. (Mnemonic: related to the -w switch.)

    $EXECUTABLE_NAME

    $^X     The name that the Perl binary itself was executed as, from
            C's `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 `$#ARGV' is the generally
            number of arguments minus one, because `$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', `require', or
            `use' constructs. 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. If you need to
            modify this at runtime, you should use the `use lib'
            pragma to get the machine-dependent library properly
            loaded also:

                use lib '/mypath/libdir/';
                use SomeMod;


    @_      Within a subroutine the array @_ contains the parameters
            passed to that subroutine. See the perlsub manpage.

    %INC    The hash %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

    $ENV{expr}
            The hash %ENV contains your current environment. Setting
            a value in `ENV' changes the environment for child
            processes.

    %SIG

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

                sub handler {    # 1st argument is signal name
                my($sig) = @_;
                print "Caught a SIG$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


            Using a value of `'IGNORE'' usually has the effect of
            ignoring the signal, except for the `CHLD' signal. See
            the perlipc manpage for more about this special case.

            The %SIG array contains values for only the signals
            actually set within the Perl script. Here are some other
            examples:

                $SIG{"PIPE"} = Plumber;     # SCARY!!
                $SIG{"PIPE"} = "Plumber";   # assumes main::Plumber (not recommended)
                $SIG{"PIPE"} = \&Plumber;   # just fine; assume current Plumber
                $SIG{"PIPE"} = Plumber();   # oops, what did Plumber() return??


            The one marked scary is problematic because it's a
            bareword, which means sometimes it's a string
            representing the function, and sometimes it's going to
            call the subroutine call right then and there! Best to
            be sure and quote it or take a reference to it. *Plumber
            works too. See the perlsub manpage.

            If your system has the sigaction() function then signal
            handlers are installed using it. This means you get
            reliable signal handling. If your system has the
            SA_RESTART flag it is used when signals handlers are
            installed. This means that system calls for which it is
            supported continue rather than returning when a signal
            arrives. If you want your system calls to be interrupted
            by signal delivery then do something like this:

                use POSIX ':signal_h';

                my $alarm = 0;
                sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
                    or die "Error setting SIGALRM handler: $!\n";


            See the POSIX manpage.

            Certain internal hooks can be also set using the %SIG
            hash. The routine indicated by `$SIG{__WARN__}' is
            called when a warning message is about to be printed.
            The warning message is passed as the first argument. The
            presence of a __WARN__ hook causes the ordinary printing
            of warnings to STDERR to be suppressed. You can use this
            to save warnings in a variable, or turn warnings into
            fatal errors, like this:

                local $SIG{__WARN__} = sub { die $_[0] };
                eval $proggie;


            The routine indicated by `$SIG{__DIE__}' is called when
            a fatal exception is about to be thrown. The error
            message is passed as the first argument. When a __DIE__
            hook routine returns, the exception processing continues
            as it would have in the absence of the hook, unless the
            hook routine itself exits via a `goto', a loop exit, or
            a die(). The `__DIE__' handler is explicitly disabled
            during the call, so that you can die from a `__DIE__'
            handler. Similarly for `__WARN__'.

            Note that the `$SIG{__DIE__}' hook is called even inside
            eval()ed blocks/strings. See the "die" entry in the
            perlfunc manpage and the "$^S" entry in the perlvar
            manpage for how to circumvent this.

            Note that `__DIE__'/`__WARN__' handlers are very special
            in one respect: they may be called to report (probable)
            errors found by the parser. In such a case the parser
            may be in inconsistent state, so any attempt to evaluate
            Perl code from such a handler will probably result in a
            segfault. This means that calls which result/may-result
            in parsing Perl should be used with extreme caution,
            like this:

                require Carp if defined $^S;
                Carp::confess("Something wrong") if defined &Carp::confess;
                die "Something wrong, but could not load Carp to give backtrace...
                     To see backtrace try starting Perl with -MCarp switch";


            Here the first line will load Carp *unless* it is the
            parser who called the handler. The second line will
            print backtrace and die if Carp was available. The third
            line will be executed only if Carp was not available.

            See the "die" entry in the perlfunc manpage, the "warn"
            entry in the perlfunc manpage and the "eval" entry in
            the perlfunc manpage for additional info.


  Error Indicators

    The variables the $@ manpage, the $! manpage, the $^E manpage,
    and the $? manpage contain information about different types of
    error conditions that may appear during execution of Perl
    script. The variables are shown ordered by the "distance"
    between the subsystem which reported the error and the Perl
    process, and correspond to errors detected by the Perl
    interpreter, C library, operating system, or an external
    program, respectively.

    To illustrate the differences between these variables, consider
    the following Perl expression:

       eval '
         open PIPE, "/cdrom/install |";
             @res = <PIPE>;
         close PIPE or die "bad pipe: $?, $!";
        ';


    After execution of this statement all 4 variables may have been
    set.

    $@ is set if the string to be `eval'-ed did not compile (this
    may happen if `open' or `close' were imported with bad
    prototypes), or if Perl code executed during evaluation die()d
    (either implicitly, say, if `open' was imported from module the
    Fatal manpage, or the `die' after `close' was triggered). In
    these cases the value of $@ is the compile error, or `Fatal'
    error (which will interpolate `$!'!), or the argument to `die'
    (which will interpolate `$!' and `$?'!).

    When the above expression is executed, open(), `<PIPE>', and
    `close' are translated to C run-time library calls. $! is set if
    one of these calls fails. The value is a symbolic indicator
    chosen by the C run-time library, say `No such file or
    directory'.

    On some systems the above C library calls are further translated
    to calls to the kernel. The kernel may have set more verbose
    error indicator that one of the handful of standard C errors. In
    such cases $^E contains this verbose error indicator, which may
    be, say, `CDROM tray not closed'. On systems where C library
    calls are identical to system calls $^E is a duplicate of $!.

    Finally, $? may be set to non-`0' value if the external program
    `/cdrom/install' fails. Upper bits of the particular value may
    reflect specific error conditions encountered by this program
    (this is program-dependent), lower-bits reflect mode of failure
    (segfault, completion, etc.). Note that in contrast to $@, $!,
    and $^E, which are set only if error condition is detected, the
    variable $? is set on each `wait' or pipe `close', overwriting
    the old value.

    For more details, see the individual descriptions at the $@
    manpage, the $! manpage, the $^E manpage, and the $? manpage.

  Technical Note on the Syntax of Variable Names

    Variable names in Perl can have several formats. Usually, they
    must begin with a letter or underscore, in which case they can
    be arbitrarily long (up to an internal limit of 256 characters)
    and may contain letters, digits, underscores, or the special
    sequence `::'. In this case the part before the last `::' is
    taken to be a *package qualifier*; see the perlmod manpage.

    Perl variable names may also be a sequence of digits or a single
    punctuation or control character. These names are all reserved
    for special uses by Perl; for example, the all-digits names are
    used to hold backreferences after a regular expression match.
    Perl has a special syntax for the single-control-character
    names: It understands `^X' (caret `X') to mean the control-`X'
    character. For example, the notation `$^W' (dollar-sign caret
    `W') is the scalar variable whose name is the single character
    control-`W'. This is better than typing a literal control-`W'
    into your program.

    All Perl variables that begin with digits, control characters,
    or punctuation characters are exempt from the effects of the
    `package' declaration and are always forced to be in package
    `main'. A few other names are also exempt:

        ENV        STDIN
        INC        STDOUT
        ARGV        STDERR
        ARGVOUT
        SIG