Perhaps WWW::Mechanize::Shell is what you are looking for? For example after fetching the page the forms command lists all forms and fields:
$ perl -MWWW::Mechanize::Shell -eshell
(no url)>get http://www.bioinformatics.nl/.../primer3plus.cgi
Retrieving http://www.bioinformatics.nl/.../primer3plus.cgi(200)
http://www.bioinformatics.nl/.../primer3plus.cgi> forms
Form [1]
POST http://www.bioinformatics.nl/.../primer3plus.cgi
(multipart/form-data) [mainForm]
... snip list of fields and default values where relevant ...