What does $mech->response->as_string return? I expect you'll find that the error originates from the server you are contacting. Presumably, you're not passing the inputs it's expecting in the request.
Well, this is Pligg registering page and as i can login after this (via web interface) i realize that it get all my data (and register me). But why it sometimes return this error i don't know.
Also i get same error on different Pligg driven site (not single one). So now i'm sure this is Pligg problem not Perl. I'll try which input i miss (may be some JavaScript).