When updating FreeBSD's ports/packages, make sure you update the dependencies of the new packages. Tools like portupgrade help with this.

If you installed any Perl modules without using ports/packages, you'll find BSDPAN has automatically converted them into packages: use pkg_info -I 'BSDPAN*' for a list.

So, finding out which modules you have installed is easy enough. Upgrading them should just be a matter of waiting for things to download and compile. This will probably cause you fewer problems than trying to retain compatibility.