I'm not sure I follow this. When you say "recompiling", do you mean recompiling the modules? Also how would one know that the new version of Perl was compiled exactly the same as the previous version? As you said, it's probably easier to just reinstall the modules.
Also in a directory where perl modules are installed locally, is a record created anywhere in that directory indicating what version of perl was used to do the installation, or do I need to maintain that myself via directory naming conventions and/or README files?