Sounds to me like your two versions of Perl have at least some directories in common between their @INC members. You can actually get away with that, but it requires really knowing what you are doing and choosing some non-default configuration options when you build the Perls that will share some of their directories. Disk has gotten much cheaper since the idea of sharing @INC directories between different installs of Perl was first considered. You are better off just making sure that each Perl has completely separate places to look for its modules.

So the best idea might be to un-install one of your versions of Perl and then re-install that version but built such that it won't have any @INC directories in common with the other version.