I was thinking that this is actually not as dumb an interview question as it at first appears to be. You could potentially learn something about your candidate's approach from how they solve it. Consider some of the things they might do:

• Write a naive solution (without even the %15 trick) very quickly. Tells you this person is good at whipping out simple code.
• Write a golfish solution. Tells you this person knows perl well, but may be too eager to show off. (An interview is probably the wrong place for golf...)
• Write an efficient solution that minimizes the number of comparisons. Tells you this person thinks about performance. Could be a danger sign for such a small program, but it is an interview.
• Write a very easy-to-read solution, with clear variable names and no tricks. Tells you this person thinks about maintenance.