My results with the Monster agent was pretty on target. The Brainbuzz one sucked, and HotJobs was OK. The agents use the same mechanisms as if you were to choose a geographical area and keyword(s) in the search
boxes. I don't think I have come across many job ads which state "All Perl, all the time. All you would ever do is Perl". Most jobs want you to also know UNIX, Apache, C, shell, DB, etc... and state that.
One good thing about the agents is that you do not have to go to the site every few days (or every day) and run the searches yourself.