Looking at the source, it looks like a very efficient implementation. It does native 64-bit arithmetic through overloaded operators. It doesn't use Math::BigInt or the like. (The number is stored in the NV slot of a blessed SV.)
...but native 64-bit arithmetic isn't supported by my compiler or it isn't enabled. Does any 32-bit compiler on a 32-bit platform have support for 64-bit arithmetic?
It would be nice if it downgraded to using Math::BigInt if necessary.