brainbench has much nastier questions. I was forced to take their test by my employer a couple of months ago when i got my new job. Most of the questions are easy, but some require standard module knowledge and funky regex stuff. Surprisingly, the test rules allow to use on-line documentation (e.g. perldoc). The amount of time you're given, anyone can get a 5.0. Just for the hell of it, I decided not to use documentation and got almost a 4 (and, ultimately, the job).

As for Perl certifications, it is a waste, IMO.