If you don't have PERL5LIB set in your environment, the problem is somewhere else.
The variable @INC is the search path where Perl looks for modules. You can inspect the module search path by running perl -V on the command line.
The value of @INC is influenced by the environment variable PERL5LIB, see perlrun and perlvar on @INC.
The link I posted explains that the environment variable PERL5LIB is ignored under taint mode.
The module lib is a convenient way to set up @INC (which PERL5LIB also sets) from within a Perl program.