Acutally in some respects i would say that unless you have a definitive need for features in 5.8 then don't bother. Some tests ive seen show a drammatic slowdown from using 5.8 that if you arent using 5.8 specific features would suggest the upgrade is unnecessary.

I would also review perl delta and look for relevant stuff. File handling changed a lot, as did some aspects of hashing. Other than that I cant really help beyond saying that you should make warnings fatal and run your code for a while to see what happens.