I'm working on a project right now that uses a lot of modules. What I did was to tuck the modules that weren't used constantly into subs. Then you can call them as methods when you need them, and you won't get out of memory warnings.

As for undef, each module will probably have its own procedure for undef or exit, so check that module's documentation to see how its done. There's a lot of modules that don't have a method for undef, but if you're using subs, you may not need to undef them.