We just did a similar update here. In fact, they neglected to tell most of their perl programmers that the update was happening, so it was a bit of a surprise for me. Still, it was a welcome upgrade.

Most things still worked. Many scripts gave minor warnings - something about // vs. "" in splits. A few larger problems.

Two options: Install 5.8 in a different path, or with a different executable name. Then run through all your scripts with /path/to/new/perl-5.8 -wc script.pl. Fix whatever errors exist. Most of it is minor stuff, so the fixes should still work in 5.005_03.

Option two: Just go for it. If nothing is very critical, just upgrade. Do the perl -wc tests on everything, and fix it as you go. Since it was a surprise, this is what I did. For a large repository of scripts here, it only took me an hour or two to fix everything. A few things broke, but most just gave extra warnings.

Good luck!
~Jon