Ah, but it isn't. $o is still associated with Tied, but the tie itself has been undone, as evidenced by the absense of a <fetch> message from the final invocation of $o->foo().

That's the oddity I've noticed about untie -- the association with the Tied class is still there (i.e., the variable is still blessed), even though the variable is no longer tied. I have yet to find this behavior documented.