We standardize on a perl verison/build and build CPAN modules against it. We will test it on our boxes first and make patches if needed. When it works we will save off the blib directory from the built module, copy it into our custom module, and store it in our repo (remember to turn off keyword expansion or any other file changing operations in your repo). Then we use a special Makefile.pl that only installs the contents the blib directory. The only conflicts we see is the .exists files.

If you want to keep the original source around you can stuff the tar.gz files into the repo. We don't do that and will go back out to CPAN for the versions we need. Few times we couldn't find a older one, but found out that the new version was better for us anyways.

And what would be a good way to determine if a previous build of perl and modules is compatible with the needs of the current app source code?"

Testing. However, unless there is some major changes to the module, then you won't find too many issues.