When things are odd with WWW::Mechanize try to debug/develop with WWW::Mechanize::Shell. It features interactive web browsing, form filling and can generate a WWW::Mechanize script from the recorded session. Here is a bit look'n feel.

perl -MWWW::Mechanize::Shell -eshell
(no url)>get http://google.de
Retrieving http://google.de(200)

http://www.google.de/>forms
Form [1]
GET http://www.google.de/search [f]
  hl=de                          (hidden readonly)
  ie=ISO-8859-1                  (hidden readonly)
  q=                             (text)
  btnG=Google-Suche              (submit)
  btnI=Auf gut Glück!            (submit)
  meta=                          (radio)    [*|lr=lang_de|cr=countryDE
+]
[download]