well you could "overload" assignments via tie to always do a copy

Yes - that might be what I'm after. I'm looking for a solution that would be applied inside Math::BigInt.
I'll see if I can get my head around it. (Thanks for the link.)

I actually struck the problem in Math::MPFR (which is where I'd like to fix it), but used Math::BigInt as the demo because everyone already has that module and I believe that it's the same issue with both of those modules.

Cheers,
Rob