Excellent Question! If I may suggest a somewhat unorthodox approach to coming up with these questions ...
in reply to A Perl aptitude test
There are plenty perl classes offered in both Universities and on the Web. It should be a moderately easy job to flip through a few "sample tests" and extract a few questions that you would feel comfortable with asking. If all you are looking for is general familiarity with the language, I think anyone who claims to know Perl on this level should be able to answer test questions from "Perl 101" and "Perl 102".