Do you mean re-factoring your own code into modules or using modules from CPAN? For the former there is probably very little overhead if done properly.

As for the latter, it depends. Modules on CPAN are often provided and maintained by very experienced programmers, so they are likely to be better than your own code. Unless your are THE expert in the field that you want to program for or you need something really special. Modules on CPAN often aim at general applicability which might create some overhead.