It's a useful convention in perl that operator-like subs (and many others) should return true if they are successful. The only one of your validation functions which might not is sub val_alpha, which successfully detaints an empty string and returns it. I make it a habit to avoid that automatically so I don't have to think about it :)

It is only a convention, but a profoundly useful one. Perl does nothing to demand you follow it.