Remember that when you're dealing with foreach loops the item that you've got ($key in this case) is the item in the hash. That is, $key becomes an alias to the memory address within the hash.

I'm not sure that's accurate. Look at the last half of Perl_do_kv in doop.c and hv_iterkeysv and hv_iterval in hv.c. (I'm looking at something around 5.8.0.) The key iterator returns a mortal copy of the key, which is pushed onto the return stack. Sure, it points to the same data, but when you're done with it, it'll be ready for garbage collection without causing the original to be collected.

The value is returned as a normal SV. I don't see any reference count increments, but there's very little you could do to cause it to be deleted from the hash without the associated key.

I could really have misunderstood what you meant, but it doesn't sound quite right.