(I assume that Devel::Size::total_size() does take into account the buckets overhead ?)

I can confirm it does. Or rather, it certainly used to back circa v0.58, when I understood it's internals enough that I succeeded in supplying a trivial patch. It seems unlikely that it would have suddenly stopped doing so now, but I pulled v0.71 earlier today and I no longer understand what hell is going on inside there.

For the rest of your post, I am unqualified to comment as I know little about *nix memory management. I have no idea what Aux is, nor even imagine what it could be. Someone with *nix experience will be needed to interprete your numbers and explain what is going on.

I do wonder if this is related to zentara's recent discovery that *nix can release some memory back to the OS under some circumstances?