You need to discover the mech-dump tool that comes with WWW::Mechanize. You feed it a file on disk, or a URL, and it tells you all the forms and fields on it.

$ mech-dump http://perlmonks.org
GET http://perlmonks.org/index.pl
  node=
  go_button=Search                (submit)  

POST http://perlmonks.org/index.pl
  node_id=131                     (hidden)  
  op=login                        (hidden)  
  user=
  passwd=                         (password) 
  expires=<UNDEF>                 (checkbox) [*<UNDEF>/off|+10y/rememb
+er me]
  login=Login                     (submit)  

POST http://perlmonks.org/index.pl
  node_id=131                     (hidden)  
  foo=Refresh                     (submit)  

POST http://perlmonks.org?
  node_id=325590                  (hidden)  
  displaytype=display             (hidden)  
  vote=<UNDEF>                    (radio)   [0/TCP|1/UDP|2/ICMP|3/IPX|
+4/SMB|5/TFTP|6/SMTP|7/NNTP|8/HTTP|9/WAP|10/SOAP]
  <NONAME>=Vote                   (submit)
[download]