Your question isn't exactly clear -- you have two different scripts, which you're running from the same computer at the same time, and they contain the same login code, but one works and the other doesn't?

Sounds unlikely.

But obviously, the sensible thing to do is just print out $mech->content() before you try to log in and examine the HTML.

Your test basically says "if there's a certain string anywhere on the page, assume I'm not logged in and try to log in". There may be something wrong with that logic -- another form on the page which looks like that, or the same form commented out? Whatever it turns out to be, the answer will almost certainly be in the HTML.