Well it will depend on the modules but if you build your own Perl part of the build/install is to search for old versions of Perl and ask if you want to add their @INC to the @INC of the new. You can still do something similar if using a package but it's more fun to build your own.

This neatly leads onto another issue. If you've done this in the past so now have several perl library sets, is there an automatted way to remove old versions of modules where newer ones exist, ie module::something v1 was installed by Perl 5.6 and is in /usr/local/lib/perl/5.6.0...., v2 was built with Perl 5.8 and is in /usr/local/lib/perl/5.8.x....

If you build the new module with the same Perl as the old it overwrites the old module, is there a nice way to remove the old version, ideally when installing the new, for building with a newer Perl?