From the documentation:

autocheck => [0|1]
Checks each request made to see if it was successful. This saves you the trouble of manually checking yourself. Any errors found are errors, not warnings.

philipbailey touched on a recent meditation about how defaulting autocheck to on broke many things for one particular monk. But in your case, you turned it on explicitly, so that monk's (mostly valid) complaints aren't applicable. You opted for autocheck yourself - and WWW::Mechanize is doing what you asked it to.