Ok, first THERE IS NO OVERFLOW. NONE, does not exist.
What you have is a vent line.. But even that terminates inside the filler neck.
Options depend on exactly where the gas was dripping.. The filler tube goes into the tank via a sealing ring. it may be leaking.
You may have a hole in your filler tube (how to get gas out of a tank with a locking gas cap)
Gas may have run out the top when the pump failed to click off
IF IT happens next time you gas up....
Have a professional inspect it
IF it does not happen the next time you fill up.. THe problem is with the pump. (Station pump) not cutting off.