This is what I came into this thread to say. Rewriting from scratch:

• Burns time you aren't sure to ever recoup with whatever perceived time-saving benefits there will be in the new version.
• Is time spent spinning your wheels, feature-wise. Customers walk away from stagnant product.
• Sacrifices a known system with known bugs for an unknown system with unknown bugs.

I'm not sure if not having Perl expertise on hand is a worthy argument either. Experience tells me that good programmers can work with anything that was written with a little care and was well documented. And if it is important enough that the slowness of the non-Perl programmer doing maintenance would be a waste, it seems likely it would be important enough to think about a new hire.