How do I add a directory to my include path (@INC) at runtime?

Here are the suggested ways of modifying your include path, including
  environment variables, run-time switches, and in-code statements:

  the "PERLLIB" environment variable
              $ export PERLLIB=/path/to/my/dir
              $ perl program.pl

  the "PERL5LIB" environment variable
              $ export PERL5LIB=/path/to/my/dir
              $ perl program.pl

  the "perl -Idir" command line flag
              $ perl -I/path/to/my/dir program.pl

  the "lib" pragma:
              use lib "$ENV{HOME}/myown_perllib";

  the "local::lib" module:
              use local::lib;

              use local::lib "~/myown_perllib";

  The last is particularly useful because it knows about machine dependent
  architectures. The "lib.pm" pragmatic module was first included with the
  5.002 release of Perl.