Explain the risks as clearly as you can, and if your boss finds the risk acceptable, proceed. As long as you are communicating, it will be a project failure, not a personal failure. Don't be combative about it, make sure everyone fails and learns together :)

Good: clearly tracking the risks as the project progresses.

Better: pushing yourself to learn Perl and mitigate the risks to the best of your ability.

Best: spend some OT implementing it in Java and show her why it is better (or show yourself why it is not).