so no "simulation" should be needed

From what's I've heard, use64bitint allows 64-bit ints in a 32-bit Perl. And as far as I know, 32-bit binaries don't support 64-bit ints without "simulation".

So you are either building a 64-bit binary (where you don't need use64bitint) or a 32-bit binary (where you need "simulation" and thus use64bitint for 64-bit ints, even on a 64-bit system).