User agent would be my first guess. Many sites (Yahoo! included) do different things if they detect a user agent that says "hi, I'm not a real person, but a program". I know this well from trying to write automated test scripts.

Use $mech->agent_alias('Windows IE 6") and space out your requests with sleep 2 or so. This will be less likely to trigger the "dude, you are so not for real" checks.