Special Variables

• The command line arguments are passed in @ARGV
• The default for shift is @ARGV in the main program.
• The input operator <> works on the files named in @ARGV by default.

• @_ is the array used to pass arguments to subroutines.
• The default for shift is @_ within a subroutine.

  foreach (@array) {
      print if /A/;
  }

  foreach $_ (@array) {
      print $_ if ($_ =~ /A/);
  }

$/ is the string that separates lines when read with the <> operator. Remember that in scalar context, <F> reads one line from the filehandle F. What constitutes a line? Whatever ends with $/. By default this is a newline, but you can set $/ to anything. In fact, if you set it to undef, you will then slurp in the whole file in scalar mode.

$! is set to an error message if a preceeding opeartion fails. Often used with die to report errors.

    open (F, "filename") or die "Can't open filename: $!";

$@ is set to a non-null error message if eval fails. Very useful for trapping exceptions.

    eval {$some_code};
    if ($@) {
	print STDERR "eval failed: $@\n";
    }

There are all sorts of other special variables, such as $| that set output buffering, or $$ that contains the current pid. But the above are the most useful to be aware of.