I think you may have missed the "...if (for example)..." part of my post?

But more than one SvNULL will be created

That's a shame. Think of the space that would be saved if all SV's that were undef, were pointers to a single, shared copy of undef with a COW flag set?

creates two SVs 36 bytes apart, which suggest you can't use anything larger than 12.

Yeah! Since posting, I looked at the addresses of consecutive elements of an array and 12 does appear to be the minimum I can rely upon.

Any thoughts on discovering the lowest and highest addresses?