Perl modules

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A module is a separate file containing Perl source code, which is loaded and executed at compile time. This means that when you write:

        use CGI;

Perl looks for a file called CGI.pm (.pm for Perl Modules), and upon finding it, loads it in and executes the code inside it, before looking at the rest of your program.

Sometimes you might choose to pass extra information to the module when you load it. Often this is to request the module create new subroutines in your namespace.

        use CGI qw(:standard);
        use File::Copy qw(copy);

Note the use of qw(), this is a list of words (in our case, just a single word). It's possible to pass many options to a module when you load it. In the case above, we're asking the CGI module for the :standard bundle of functions, and the File::Copy module for just the copy subroutine.

Module Uses

Perl modules can do just about anything. In general, however, there are three main uses for modules:

• Changing how the rest of your program is interpreted. For example, to enforce good coding practices (use strict) or to allow you to write in other languages, such as Latin (use Lingua::Romana::Perligata), or to provide new language features (use Switch).
• To provide extra functions to do your work (use Carp or use CGI qw/:standard/).
• To make available new classes (use HTML::Template or use Finance::Quote) for object oriented programming.

Sometimes the boundaries are a little blurred. For example, the CGI module provides both a class and the option of extra subroutines, depending upon how you load it.

Where does Perl look for modules?

Perl searches through a list of directories that are determined when the Perl interpretor is compiled. You can see this list (and all the other options Perl was compiled with), by using perl -V.

The list of directories which Perl searches for modules is stored in the special variable @INC. It's possible to change @INC so that Perl will search in other directories as well. This is important if you have installed your own private copy of some modules.

Of course, being Perl, there's more than one way to change @INC. Here are some of the ways to add to the list of directories inside @INC:

• Call Perl with the -I command-line switch with the location of the extra directory to search. For example:

          perl -I/path/to/libs

  This can be done either in the shebang line, or on the command-line.

• Use the lib pragma in your script to inform Perl of extra directories. For example:

          use lib "/path/to/libs";

• Setting the PERL5LIB environment variable with a colon-separated list of directories to search. Note that if your script is running with taint checks this environment variable is ignored.

          PERL5LIB=/path/to/libs:/other/path/to/libs

Since use statements occur before regular Perl code is executed, modifying @INC directly usually does not have the desired effect.

Finding installed modules

To find out which modules were included standard with your Perl installation type:

        perldoc perlmodlib

To find out which modules have since been installed type:

        perldoc perllocal

This is not fool-proof as many operating system packaging systems do not update the file used to provide this information. Further, modules installed by users without permission to update this file will also be missing.

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