adrianh said I wasn't clear about what this code shows

I meant to say I didn't understand what you were getting at - apologies :-)

If I had a way of comparing past object identity with present objects then I could assert $s1_addr != Class::WeakSingleton->instnace.

If it were me I would check that $s1 had been deallocated using weak references. Something like this example. What you want to test (if I'm reading this correctly) is that the object goes away when it falls out of scope. Having a unique object identifier isn't the only way of doing this.

It turns out that there's a much easier solution which just has me assigning a unique value to each object

Not that this is a bad solution either :-)