If there are holes in your test scripts, then they are not testing everything you deem important! If you can't trust your test suite to test your scripts and modules, then what can you trust? A hit-and-miss perl -c approach will be unlikely to pick up any more bugs than your tests. If you don't think your tests are good enough, I'd get intimate with the relevant incompatible changes in the perldeltas (a quick search on your scripts for threads and unicode might not go amiss); then install your modules and scripts on a dev box, and run them into the ground with automated tests. Whenever you find a bug, add it to the test suite, and that's one less thing you need to do manually next time. Best of luck with it.
in reply to Perl Version Change - Detecting Problems in Advance