To me; prototyping keeps me from making subtle design changes (just 1 extra arg, in this special case) that are not documented, and probably will cause maintenance problems later.

If you're doing that, you don't understand what prototypes do (and this is common, and one of the reasons for the general rule that you shouldn't use them unless you've got a good reason).

Prototypes in perl convert arguments and allow short-cuts in the calling code. The "checking" of the arguments only happens as a side effect, and you'll only get warnings/errors if the arguments cannot get converted. This probably means you'll have subtler bugs, not less.

See also Are prototypes evil?, for example