Run shared-threaded or under mod_perl, instance 1 gets the lock. Instance 2 opens the global handle *SEM. Instance 1's handle is closed and its lock gone, but since it knows the global handle only by name, it never learns of it.

Update: The two don't know about each other, are sharing a filehandle, and are executing the same code at maybe the same time. Globals are extra evil in such an environment.