I think your fear of rewriting from scratch is quite good. Since you have new development work to do, now is a great time to start writing tests. I wouldn't write tests for the whole system, just the parts you touch. When you've tested an individual piece sufficiently, take a few minutes to clean it up.

If the important parts of the program are in the bit that creates reports, you'll have tested and cleaned the most important parts of the program once you're done. With those tests in place, it'll be much easier to port the program to Perl 5.8.

You can find more information in Five Lessons You Should Learn From Extreme Programming and Five Lessons Open Source Developers Should Learn From Extreme Programming.