Resove the problem of @INC of perl.

1. Default @INC

   Perl interpreter is compiled with a specific default value of @INC that it was compiled with. To find out this value, runenv -i perl -V command (`env -i ignores PERL5LIB environmental variable - see #2) and in the output you will see something like this:

2. $ perl -V...%ENV:  PERL5LIB="/home/myuser/test"@INC:  /home/myuser/test /usr/local/lib/perl5/5.8.6/i686-linux /usr/local/lib/perl5/5.8.6 /usr/local/lib/perl5/site_perl/5.8.6/i686-linux /usr/local/lib/perl5/site_perl 

   
   

3. -I command line parameter

   Perl pre-pends @INC with a list of directories (colon-separated) passed in to it as a value of-I command line parameter. This can be done in one of two ways, as usual with Perl parameters:

   • Pass it on command line: perl -I /my/moduledir your_script.pl

   • Pass it via the first (shebang) line of your Perl script:

   #!/usr/local/bin/perl -w -I /my/moduledir

   • Pass it as part of PERL5OPT (or PERLOPT) environmantal variable (see chapter 19.02 in Programming Perl)

4. Pass it via the use lib pragma

   Perl pre-pends @INC with a list of directories passed in to it viause lib pragma.

4. In a program:

   use lib ("/dir1", "/dir2")

   On the command line:

   perl -Mlib=/dir1,/dir2

   You can also remove the directories from @INC via no lib

5. You can directly manipulate @INC as a regular Perl array.

   NOTE: Since @INC us used during compilation phase, this must be done inside of aBEGIN {} block, which precedes the use MyModule statement;

   • Add directories to the beginning via unshift @INC, $dir

   • Add directories to the end via push @INC, $dir

   • Do anything else you can do with a Perl array.

NOTE The directories are unshifted onto @INC in the order listed in this answer, e.g. default @INC is last in the list, preceded by PERL5LIB, preceded by -I, preceded by "use lib" and direct @INC manipulation, the latter two mixed in whichever order they are in Perl code.

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We will look at how the contents of this array are constructed and can be manipulated to affect where Perl interpreter will find the module files.

1. Default @INC

   Perl interpreter is compiled with a specific default value of @INC that it was compiled with. To find out this value, runenv -i perl -V command (`env -i ignores PERL5LIB environmental variable - see #2) and in the output you will see something like this:

   $ env -i perl -V...@INC:  /usr/local/lib/perl5/5.8.6/i686-linux /usr/local/lib/perl5/5.8.6 /usr/local/lib/perl5/site_perl/5.8.6/i686-linux /usr/local/lib/perl5/site_perl 

   To change the default path when configuring Perl binary compilation, use otherlibdirs variable:

   Configure -Dotherlibdirs=/usr/lib/perl5/site_perl/5.8.1

2. Environmental variable PERL5LIB (or PERLLIB)

   Perl pre-pends @INC with a list of directories (colon-separated) contained inPERL5LIB (if it is not defined, PERLLIB is used) environmental variable of your shell. To see the contents of @INC after PERL5LIB and PERLLIB environment variables have taken effect, runperl -V.

The directories are unshifted onto @INC in the order listed in this answer, e.g. default @INC is last in the list, preceded by PERL5LIB, preceded by -I, preceded by "use lib" and direct @INC manipulation, the latter two mixed in whichever order they are in Perl code.

References:

• perldoc perlmod
• perldoc lib
• Perl Module Mechanics - a great guide containing practical HOW-TOs
• How do I 'use' a Perl module in a directory not in@INC?
• Programming Perl - chapter 31 part 13, ch 7.2.41
• How does a Perl program know where to find the file containing Perl module it uses?

There does not seem to be a comprehensive "@INC" FAQ-type post on StackOverflow, so this question is intended as one.

When to use each approach?

• 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 default @INC compiled into Perl binary

• 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 @INC in previous use case), set the users'PERL5LIB, usually during user login.

  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. viasu.

• 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 PERL5LIB manually, or use "-I" perl parameter.

• If the modules need to be used only for specific scripts, by ALL users using them, useuse lib/no lib 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 script's command line parameters or script's path (see FindBin module for very nice use case)

• If the directories in @INC need to be manipulated according to some complicated logic, either impossible to too unwieldy to implement by combination of "use lib/no lib" pragmas, then use direct@INC manipulation inside BEGIN {} block or inside a special purpose library designated for @INC manipulation, which must be used by your script(s) before any other modules are used.

  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 this SO post.

• An additional use case for directly manipulating @INC is to be able to add subroutine references or object references (yes, Virgina,@INC can contain custom Perl code and not just directory names, as explained here)

Original Link: http://stackoverflow.com/questions/2526804/how-is-perls-inc-constructed-aka-what-are-all-the-ways-of-affecting-where-pe