May be this is JavaScript? Try to clear all site specifiec cookies in the browser than disable JavaScript and after that make a request to the site again (in the browser). If you get results than you'll know that this is not JavaScript.

If it not JavaScript than it can be any file (image, css) which target page refer (you can find it by Set-Cookie header in the session log.

UPDATE:and i see LWP::UserAgent but not WWW::Mechanize in your code! Life is short so use WWW::Mechanize!