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    <p>We will look at how the contents of this array are constructed and can be manipulated to affect where the Perl interpreter will find the module files.</p> <ol> <li><p>Default <code>@INC</code></p> <p>Perl interpreter is <a href="http://search.cpan.org/perldoc/INSTALL" rel="noreferrer">compiled with a specific <code>@INC</code> default value</a>. To find out this value, run <code>env -i perl -V</code> command (<code>env -i</code> ignores the <code>PERL5LIB</code> environmental variable - see #2) and in the output you will see something like this:</p> <blockquote> <pre><code>$ env -i perl -V ... @INC: /usr/lib/perl5/site_perl/5.18.0/x86_64-linux-thread-multi-ld /usr/lib/perl5/site_perl/5.18.0 /usr/lib/perl5/5.18.0/x86_64-linux-thread-multi-ld /usr/lib/perl5/5.18.0 . </code></pre> </blockquote> <p>Note <code>.</code> at the end; this is the current directory. It is missing when Perl runs with <a href="http://perldoc.perl.org/perlrun.html#*-T*" rel="noreferrer"><code>-T</code> (taint checks enabled)</a>.</p> <p>To change the default path when configuring Perl binary compilation, set the configuration option <a href="http://search.cpan.org/perldoc?INSTALL#otherlibdirs" rel="noreferrer"><code>otherlibdirs</code></a>:</p> <blockquote> <p><code>Configure -Dotherlibdirs=/usr/lib/perl5/site_perl/5.16.3</code></p> </blockquote></li> <li><p>Environmental variable <code>PERL5LIB</code> (or <code>PERLLIB</code>)</p> <p>Perl pre-pends <code>@INC</code> with a list of directories (colon-separated) contained in <code>PERL5LIB</code> (if it is not defined, <code>PERLLIB</code> is used) environment variable of your shell. To see the contents of <code>@INC</code> after <code>PERL5LIB</code> and <code>PERLLIB</code> environment variables have taken effect, run <code>perl -V</code>.</p> <blockquote> <pre><code>$ perl -V ... %ENV: PERL5LIB="/home/myuser/test" @INC: /home/myuser/test /usr/lib/perl5/site_perl/5.18.0/x86_64-linux-thread-multi-ld /usr/lib/perl5/site_perl/5.18.0 /usr/lib/perl5/5.18.0/x86_64-linux-thread-multi-ld /usr/lib/perl5/5.18.0 . </code></pre> </blockquote></li> <li><p><code>-I</code> command-line option</p> <p>Perl pre-pends <code>@INC</code> with a list of directories (colon-separated) passed as value of the <code>-I</code> command-line option. This can be done in three ways, as usual with Perl options:</p> <ul> <li><p>Pass it on command line:</p> <pre><code>perl -I /my/moduledir your_script.pl </code></pre></li> <li><p>Pass it via the first line (shebang) of your Perl script:</p> <pre><code>#!/usr/local/bin/perl -w -I /my/moduledir </code></pre></li> <li><p>Pass it as part of <code>PERL5OPT</code> (or <code>PERLOPT</code>) environment variable (see chapter 19.02 in <a href="http://oreilly.com/catalog/9780596004927" rel="noreferrer">Programming Perl</a>)</p></li> </ul></li> <li><p>Pass it via the <a href="http://perldoc.perl.org/lib.html" rel="noreferrer"><code>lib</code> pragma</a></p> <p>Perl pre-pends <code>@INC</code> with a list of directories passed in to it via <code>use lib</code>.</p> <p>In a program:</p> <pre><code>use lib ("/dir1", "/dir2"); </code></pre> <p>On the command line:</p> <pre><code>perl -Mlib=/dir1,/dir2 </code></pre> <p>You can also <a href="http://perldoc.perl.org/lib.html#Deleting-directories-from-@INC" rel="noreferrer">remove the directories from <code>@INC</code> via <code>no lib</code></a>.</p></li> <li><p>You can directly manipulate <code>@INC</code> as a regular Perl array.</p> <p>Note: Since <code>@INC</code> is used during the compilation phase, this must be done inside of a <code>BEGIN {}</code> block, which precedes the <code>use MyModule</code> statement.</p> <ul> <li><p>Add directories to the beginning via <code>unshift @INC, $dir</code>.</p></li> <li><p>Add directories to the end via <code>push @INC, $dir</code>.</p></li> <li><p>Do anything else you can do with a Perl array.</p></li> </ul></li> </ol> <p>Note: The directories are <em>unshifted</em> onto <code>@INC</code> in the order listed in this answer, e.g. default <code>@INC</code> is last in the list, preceded by <code>PERL5LIB</code>, preceded by <code>-I</code>, preceded by <code>use lib</code> and direct <code>@INC</code> manipulation, the latter two mixed in whichever order they are in Perl code.</p> <h3>References:</h3> <ul> <li><a href="http://perldoc.perl.org/perlmod.html#Perl-Modules" rel="noreferrer">perldoc perlmod</a></li> <li><a href="http://perldoc.perl.org/lib.html" rel="noreferrer">perldoc lib</a></li> <li><a href="http://world.std.com/~swmcd/steven/perl/module_mechanics.html" rel="noreferrer">Perl Module Mechanics - a great guide containing practical HOW-TOs</a></li> <li><a href="https://stackoverflow.com/questions/185114/how-do-i-use-a-perl-module-in-a-directory-not-in-inc">How do I 'use' a Perl module in a directory not in <code>@INC</code>?</a></li> <li><a href="http://rads.stackoverflow.com/amzn/click/0596000278" rel="noreferrer">Programming Perl</a> - chapter 31 part 13, ch 7.2.41</li> <li><a href="https://stackoverflow.com/questions/2526520/how-does-a-perl-program-know-where-to-find-the-file-containing-perl-module-it-use">How does a Perl program know where to find the file containing Perl module it uses?</a></li> </ul> <p>There does not seem to be a comprehensive <code>@INC</code> FAQ-type post on Stack Overflow, so this question is intended as one.</p> <h3>When to use each approach?</h3> <ul> <li><p>If the modules in a directory need to be used by many/all scripts on your site, especially run by multiple users, that directory should be included in the default <code>@INC</code> compiled into the Perl binary.</p></li> <li><p>If the modules in the directory will be used exclusively by a specific user for all the scripts that user runs (or if recompiling Perl is not an option to change default <code>@INC</code> in previous use case), set the users' <code>PERL5LIB</code>, usually during user login.</p> <p><em>Note: Please be aware of the usual Unix environment variable pitfalls - e.g. in certain cases running the scripts as a particular user does not guarantee running them with that user's environment set up, e.g. via <code>su</code>.</em></p></li> <li><p>If the modules in the directory need to be used only in specific circumstances (e.g. when the script(s) is executed in development/debug mode, you can either set <code>PERL5LIB</code> manually, or pass the <code>-I</code> option to perl.</p></li> <li><p>If the modules need to be used only for specific scripts, by <em>all</em> users using them, use <code>use lib</code>/<code>no lib</code> pragmas in the program itself. It also should be used when the directory to be searched needs to be dynamically determined during runtime - e.g. from the script's command line parameters or script's path (see the <a href="http://p3rl.org/FindBin" rel="noreferrer">FindBin</a> module for very nice use case).</p></li> <li><p>If the directories in <code>@INC</code> need to be manipulated according to some complicated logic, either impossible to too unwieldy to implement by combination of <code>use lib</code>/<code>no lib</code> pragmas, then use direct <code>@INC</code> manipulation inside <code>BEGIN {}</code> block or inside a special purpose library designated for <code>@INC</code> manipulation, which must be used by your script(s) before any other modules are used.</p> <p>An example of this is automatically switching between libraries in prod/uat/dev directories, with waterfall library pickup in prod if it's missing from dev and/or UAT (the last condition makes the standard "use lib + FindBin" solution fairly complicated. A detailed illustration of this scenario is in <a href="https://stackoverflow.com/questions/2474588/how-do-i-use-beta-perl-modules-from-beta-perl-scripts"><em>How do I use beta Perl modules from beta Perl scripts?</em></a>.</p></li> <li><p>An additional use case for directly manipulating <code>@INC</code> is to be able to add subroutine references or object references (yes, Virginia, <code>@INC</code> can contain custom Perl code and not just directory names, as explained in <a href="https://stackoverflow.com/questions/2541119/when-is-a-subroutine-reference-in-inc-called"><em>When is a subroutine reference in @INC called?</em></a>).</p></li> </ul>
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    1. POHow is Perl's @INC constructed? (aka What are all the ways of affecting where Perl modules are searched for?)
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    1. COdon't forget PERLOPT, where you can set -I. Also, things such as base.pm and local::lib use the things you've listed implicitly.
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    2. CO@brian - use::base uses it due to "require" underneath, IIRC. I'm not familiar with local::lib, will need to read more to understand what it is about
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    3. COAlso, to make this a really good answer, you need to tell people when they should use each one. Just giving them 10 options on how they could isn't very helpful. :)
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