Precedence

     Perl operators have the  following  associativity  and  pre-
     cedence:

     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 opera-
     tors  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  argu-
     ments 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  parame-
     ters.   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  sub-
     routine.  To create local variables see the local operator.

     A subroutine is called using the do operator or the & opera-
     tor.

     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