I think in some cases its acceptable to say that you don't know perl but you're interested in learning it. When I interviewed for my current job, I'd only written one perl program (and not a very complicated one), and didn't mention perl on my resume. I was asked about perl and was able to say that I was very interested in learning it and had started doing so. I went on to talk about some of the features of the language that I liked, comparing it to other languages (which probably won't work if the person interviewing you doesn't code). I like to think that I showed some understanding of perl, even if I hadn't really used it. I got the job, at any rate, about one week after writing my first perl program. Of course you can spend a few months learning more before looking for a job, but it's also quite fun to get paid while you're learning. You're never short of projects, and almost every project requires that you learn something new, and oftentimes there's someone more experienced around who can take a look at your code before implementation and offer suggestions for improvement.

You should definately use the fact that you know other languages to your advantage. I imagine you remember how much faster it was to get a working knowledge of the second language than the first. Mention that you've done similar tasks with other languages (if you have), and remind people that knowing how to code can be a lot harder than gaining a working knowledge of any language. Supposing in the future they want some other language--the person who is capable of learning other languages is a useful person (I know, you wouldn't want to stick with a company that would forsake Perl, but you can also apply this concept to updates of Perl).