in reply to
Log In To guardian.co.uk with WWW::Mechanize
Notice that your script is somehow sending you to a page in "mydetails", wich doesn't happen with the normal login process (take a look at the LiveHTTP log, you are not at any point being redirected to any page on mydetails directory).
So, for some reason, you are trying to post your data to a page that doesn't support the POST method, wich I think is done by some kind of "protection" against this kind of method.
I've tried playing with WWW::Mechanize once and I've found out that while I could easily use it to send an email on my site contact page, I couldn't use it to to manipulate other sites.