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 ...
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