I'd like to see some evidence.

Of the various good arguments for using CGI.pm, this one has always struck me as the weakest. The use of a semicolon as an argument delimeter is so effectively deprecated as to be non-existent. When is the last time you heard of any agent using a semicolon as a delimeter (other than a hand-rolled agent whose purpose is to demonstrate that it can be done)? What browsers use a semicolon?

I know of a few public web-based systems that have run for years without encountering a semicolon used as a parameter delimeter. Is that proof that such a thing can never happen? No. Are these systems at risk? No, unless some practical joker decides to hand-roll a request.