Perl's %INC hash is used to cache the names of the files and modules that were loaded and compiled by use( ), require( ), or do( )statements. Every time a file or module is successfully loaded, a new key-value pair is added to %INC. The key is the name of the file or module as it was passed to one of the three functions we have just mentioned. If the file or module was found in any of the @INC directories (except "."), the filenames include the full path. Each Perl interpreter, and hence each process under mod_perl, has its own private %INC hash, which is used to store information about its compiled modules.

Before attempting to load a file or a module with use( ) or require( ), Perl checks whether it's already in the %INC hash. If it's there, the loading and compiling are not performed. Otherwise, the file is loaded into memory and an attempt is made to compile it. Note that do( ) loads the file or module unconditionally—it does not check the %INC hash. We'll look at how this works in practice in the following examples.

First, let's examine the contents of @INC on our system:

panic% perl -le 'print join "\n", @INC'

Notice . (the current directory) as the last directory in the list.

Let's load the module and see the contents of %INC:

panic% perl -le 'use strict; print map {"$_ => $INC{$_}"} keys %INC' => /usr/lib/perl5/5.6.1/

Since was found in the /usr/lib/perl5/5.6.1/ directory and /usr/lib/perl5/5.6.1/ is a part of @INC, %INC includes the full path as the value for the key

Let's create the simplest possible module in /tmp/


This does absolutely nothing, but it returns a true value when loaded, which is enough to satisfy Perl that it loaded correctly. Let's load it in different ways:

panic% cd /tmp
panic% perl -e 'use test; \
       print map { "$_ => $INC{$_}\n" } keys %INC' =>

Since the file was found in . (the directory the code was executed from), the relative path is used as the value. Now let's alter @INC by appending /tmp:

panic% cd /tmp
panic% perl -e 'BEGIN { push @INC, "/tmp" } use test; \
       print map { "$_ => $INC{$_}\n" } keys %INC' =>

Here we still get the relative path, since the module was found first relative to ".". The directory /tmp was placed after . in the list. If we execute the same code from a different directory, the "." directory won't match:

panic% cd /
panic% perl -e 'BEGIN { push @INC, "/tmp" } use test; \
       print map { "$_ => $INC{$_}\n" } keys %INC' => /tmp/

so we get the full path. We can also prepend the path with unshift( ), so that it will be used for matching before ".". We will get the full path here as well:

panic% cd /tmp
panic% perl -e 'BEGIN { unshift @INC, "/tmp" } use test; \
       print map { "$_ => $INC{$_}\n" } keys %INC' => /tmp/

The code:

BEGIN { unshift @INC, "/tmp" }

can be replaced with the more elegant:

use lib "/tmp";

This is almost equivalent to our BEGIN block and is the recommended approach.

These approaches to modifying @INC can be labor intensive: moving the script around in the filesystem might require modifying the path.