I was told that they do not punch the hole in your old passport anymore if you renew your passport online. Members on this forum have shared their experiences, you don't even have to send the old passport back. I guess they just invalidate your old passport in their system making you unable to use your old passport.
EDIT: Also when you report the loss of your passport, they also immediately invalidate it. This is especially helpful and important especially in cases of passport theft.