|Think about Loose Coupling|
Most if not all certificates I've seen are for some large system vendors offer (be that system built upon open source components or not), ostensibly to make sure customers get the product handled by contractors who know it, and besides that got knowledge of intrinsics which the vendor chooses to sell by way of those certificates.
But the real reason is to be able to shell out 1st and 2nd level support to those contractors, while still making money of that. Nice business model.
Computer languages don't fall into that category. You get a CS degreee, and that is all there is to it. Ever saw a certificate that stamps you as "certified C programmer" which is more than a written congratulation for having passed a course at some training insistute? Ever heard of "certified PostScript programmers" or "certified FORTH professionals" or the like?
dotNet is a different thing, it's Microsoft. They want to make money, and these certificates are just for those who literally buy into Microsoft's system of making bucks, nothing more. You can't tell by way of a dotNet certificate whether someone does really have any programing skills, let alone (broad) knowledge of the field in which they are supposed to excercise their skills.
A "perl certificate" doesn't make any sense. There would be such a thing around for long now if it really did. And speaking of dotNet again - no, there will be no MONO certificate, I guess...
Those that require a certificate will never consider using perl, since there's no company behind that language which they can sue if things go awry, there are no "integrated tools" sold along, no support, no liability, in short, perl is no "product".
update: fixed several typos, but left one in place: insistute... :-D