While writing these tests I found out that I can not submit a form using WWW::Mechanize with an invalid entry entered into a radio button field. WWW::Mechanize uses the HTML::Form module to handle form parsing/filling/submitting, and that module happens to be very strict about the values you can assign to a field that is a radio button (or a select box for that matter)
Well, what do you expect your Web Browser to behave on radio boxes? Will the browser allow you to select some non-existant radio button? WWW::Mechanize is imitating what a web browser is doing - by not allowing you to make invalid selections. Somehow I fail to see why you would want to do invalid radio button selection, sounds more like hacking to me. ;-)