You might point your manager at On Not Rewriting by Joel Spolsky.

From a business point of view, rewriting working code (without changing its function) doesn't make sense: you are spending lots of time and money without improving the user experience. Indeed, the user experience may get a lot worse if you introduce new bugs in the rewrite. After all, the user doesn't usually care what language a product is written in. Would this time and money be better spent in adding new customer-requested features? If your manager spends a lot of money rewriting your working code in Cold Fusion, adds no new features, and (as is likely) introduces new bugs in the process, he will start to smell very badly.

See also:

• How to retain perl in-house code
• Defending Perl