Here or elsewhere, read up about this thing called @INC.   This is the list of places that Perl looks.   Other good search keys are PERL5LIB.   Entering perl -V on the command line (with an upper-case “V”) is also very informative.

Also, very specifically, perldoc perlfaq8 where you will find a section specifically named, How do I add a directory to my include path (@INC) at runtime?