My approach to language design has always been that people should learn just enough of the languages to get their jobs done. They shouldn't have to learn the whole language to begin with. But with certification, you have to be learning the whole language. Some people feel more comfortable that way. I guess if you want to hire experts, you want to make sure they're experts.
Certification is useful for that.
But most of the programming out there is not done by Perl experts. It's mostly done by Perl novices, and they sometimes make sloppy programs, that's ok. They learn by experience to do better over time and eventually they become experts and then, if they want to get certified and somebody wants to certify them, that's fine. I just don't want to do that myself.