Hmm, I know it's not Perl, but in my opinion form validation on the web is faster and easier to do in simple cases like this using Javascript.

Then you don't have to process the form on the server just to see if the user filled in one field OR another. The web browser does this and can display a warning message on the screen as well without any traffic to the server being generated.

Simple functionality like this should be fairly browser-independent. If you are targetting browsers that don't support Javascript, though, use Perl;