If I understand correctly, you can think of it as roughly akin to a lexical glob assignment. So the lvaluable (lvalued?) method is called on each assignment to $thresh. The moon stays the same, but there are several fingers, each with a different name.
Comment on Re: (MeowChow) Re8: Comprehending Binary :=
The method gets called when the binding is first done.
At that point the $thresh variable is bound to the lvalue
returned by the method call. Thereafter any assignment to
$thresh assigns the the original lvalue,
which has the effect of assigning to the result of
the method call, but
without the cost of the method call.