Last week we discussed the input record separator variable ($/), one of Perl’s special global variables. Perl has a large number of special variables (all listed and explained in the perlvar manpage), but you really only need to be familiar with a handful (or two) for most programming requirements. The following subset lists the most common such variables, the remainder can be looked up in perlvar as needed.

    $_      The default variable (often used, seldom seen)
    $/      Input record separator (default is "\n")
    $\      Output record separator (default is "")
    $,      Output field separator (default is "")
    $"      Field separator for interpolated arrays (default is " ")

    $|      Autoflush variable for currently selected filehandle
    $ARGV   Name of current file being read by <ARGV>
    $.      Current line number being read

    $0      The name of the current script
    $$      The current process id

    $1..$N  Captured data in regular expressions

A couple of things to be aware of: $ARGV provides the name of the current file being read via <ARGV> — and you should remember that whenever you do something like:

    while (<>) {
        print;
    }

The empty <> is either reading from STDIN, or ARGV (the latter if there were any arguments in the @ARGV array). When reading from ARGV, the $ARGV variable will be set to each filename in turn. Also, when reading from ARGV, the $. variable does not automatically reset between files, so it will represent the current total line number (see the documentation for the eof() function to work around this).

There are also some special array and hash variables you need to know:

    @ARGV   The command line argument array
    @_      The subroutine argument array

    %ENV    Hash of environment variables
    %INC    Hash of filenames that have been included (via do(),
            require() or use).
    @INC    Search paths to find included files

    @EXPORT     List of things to export from a module by default
    @EXPORT_OK  List of things to export from a module on demand
    @ISA        Inheritance

The latter ones listed above are only relevant for creating modules, and you shouldn’t need them for general programming. The @INC array lets Perl know where to look modules or libraries that you include via ‘use’, do(), or require(). By default it holds all the necessary paths created when perl itself was built and installed (which is where most new modules will be installed as well). Sometimes you need to install modules in non-standard places and you will need to be able get these paths into the @INC array so perl can find them.

There are a couple of ways you can modify the @INC array. The first is to use the PERL5LIB environment variable — if that is defined when you start your script, then the paths defined will be prepended to the @INC array. To add paths to this array within your script, you can use the ‘use lib’ pragma:

    #!/usr/bin/perl -w
    use strict;
    use lib qw(/home/jandrew/perl/lib);
    use MyModule;
    ...

The above first prepends the path ’/home/jandrew/perl/lib’ to the @INC array, then the call to ‘use MyModule’ will search that path first to try to locate the module in question.

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