neversaint,
This sounds like it might be a great (ab)?use of a Code::Critic policy. I do not know of any conversion routines and, quite frankly, I wouldn't trust them if they did exist.

Simply correcting issues identified by perl -Mstrict -wc script.pl is not going to cut the mustard but it is probably a good start. The problem arises when the code is valid (it compiles) but does not behave the same. That is why you should have a test-suite that works with the oldest version of Perl you intend to support.