As someone who gives Perl phone screens, I can tell you it is hard find good questions to ask over the phone. They have to be simple enough to be asked and answered verbally, but deep enough to see if they actually know anything.

The question, as you have explained it, strikes me as a fairly reasonable one, not so different from many I ask myself. With so many web designers trying to fake their way into Perl programming positions (~%40 of my phone screens), questions like this that draw on experience, rather than what they booked up on last night, are invaluable. I wouldn't expect a particularly good design answer off the top of someone's head, but it is not unreasonable to expect something.

Remember the purpose of an interview (or mine, at least) isn't to trip you up on things you don't know. It is to give you an oppurtunity to show what you do know. And for a good candidate, open ended questions like yours are pure gold.