What is this module "mechanise::firefox" and what capabilities does it offer to close windows? Maybe you meant "WWW::Mechanize::Firefox, but I'm unaware of any documented function in it that closes windows. What part of the documentatfion leads you to believe that there is such a functionality and what code did you try to use it?

Update: It has been pointed out that the "window" is a modal page element. Then using the ->click method or whatever other user interaction that modal page elment affords could be used.