I think your solution is just fine. Otherwise, you could use something like WWW::Form and set the 'optional' attribute on its validators as needed.

Update: Didn't notice that you were using symbolic refs. That makes your solution less fine (:

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I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated