It belongs in HTML::Form, where the enforcement of hidden fields is. However, it must be optional. The hacky way of turning off the hidden field behavior of HTML::Form is the most irritating thing about Mechanize.
Comment on Re: RFC: Where to patch to enforce maxlength in Mech?
I thought about it some more and just went w/a patch that adds a warning. That would help at least identify times when this is quietly causing a problem (like the post that started this) and is a nice easy, straight-forward, non-intrusive patch.
Patch: http://rt.cpan.org//Ticket/Display.html?id=32239