Perl FAQ 8 offers the following, under the question of:

How do I add a directory to my include path at runtime?

Here are the suggested ways of modifying your include path:

   the PERLLIB environment variable
   the PERL5LIB environment variable
   the perl -Idir command line flag
   the use lib pragma, as in
      use lib "$ENV{HOME}/myown_perllib";

The latter 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.

Myself, I go for the perl -Idir version.