|Perl Monk, Perl Meditation|
While <readmore> tags are helpful, I think you would have seen more response if you had provided the minimalist test case as that makes it much easier to separate the wheat from the chaff.
It also isn't as difficult as you seem to think: make a copy of the problem page (or code) and start hacking away bits of the copy.
For the HTML you posted, that would have resulted in something like...
It really limits where the problem might be when you only have 3 lines of HTML returning the same error as the 371-line page you pointed people to. :-)
Anyhoo, your problem:-
WWW::Mechanize (well, HTML::Form actually) only supports "submit" and "image" type buttons for clicking, and not the "button" type buttons that are used on your form.
The reason for this is "button" type buttons are meant for use with JavaScipt or some other client-side scripting, which WWW::Mechanize doesn't support.
Fortunately, there's an easy workaround, which you can find details and examples of by Super Searching for "update_html".