Special Variables

`$_`

$_ is the "it" variable. It's often the default parm that built-in functions use, or return into.

    while ( <> ) {  # Read a line into $_
        print lc;   # print lc($_)
    }

This is the same as

    while ( $it = <> ) {
        print lc($it);
    }

`$0`

$0 contains the name of the program being run, as given to the shell. If the program was run directly through the Perl interpreter, $0 contains the file name.

    $ cat file.pl
    #!/usr/bin/perl
    print $0, "\n";
    $ ./file.pl
    file.pl
    $ perl file.pl
    file.pl
    $ perl ./file.pl
    ./file.pl
    $ cat file.pl | perl
    -

$0 is what C programmers would expect to find as the first element of the argv array.

@ARGV

@ARGV contains the arguments given to the program, as ordered by the shell.

    $ perl -e 'print join( ", ", @ARGV), "\n"' 1 2 3
    1, 2, 3
    $ perl -e 'print join( ", ", @ARGV), "\n"' 1 "2 3" 4
    1, 2 3, 4

C programmers may be confused, since $ARGV[0] is really their argv[1]. Don't let this mistake happen to you!

`@INC`

@INC contains all the paths that Perl will search to find a module.

Perl programmers used to append or prepend to @INC to add a library path; these days, use lib is used instead. The following are mostly equivalent:

    BEGIN { unshift @INC, "local/lib" };

    use lib "local/lib";

%ENV

%ENV contains a copy of the current environment. It is the environment that will be given to any subshell created by Perl.

This is significant in taint mode, as %ENV will have entries that can alter the shell's behavior. For that reason, perlsec recommends the following code be used prior to executing a command in taint mode:

    $ENV{'PATH'} = '/bin:/usr/bin'; # change to your real path
    delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};

`%SIG`

Perl has rich signal handling capabilities; using the %SIG variable, you can make any subroutine run when a signal is sent to the running process.

This is especially useful if you have a long-running process, and would like to reload configuration files by sending a signal (usually SIGHUP) instead of having to start and stop the process.

You can also change the behavior of die and warn by assigning to $SIG{__DIE__} and $SIG{__WARN__}, respectively.

`<>`

The "diamond operator", <> is used when a program is expecting input, but isn't concerned how it arrives.

If the program receives any arguments, they are taken to be file names and their contents are sent through <>. Otherwise, standard input (STDIN) is used.

<> is especially useful for filter programs.

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