A factor 40 of slowdown? Gee... I would think you have forgotten to enable one of the speed boost libraries. By default, Math::BigInt and Math::BigFloat use the Math::BigInt::Calc library, which is in pure perl...!

Math::BigInt::FastCalc and Math::BigInt::BitVect ( + Bit::Vector) seem like better alternatives to me, as well as Math::BigInt::Pari and Math::BigInt::GMP (I'm just quoting from the docs now, I have no experience with any of the latter). Enough choice for alternatives, I would say.

I do wonder why a pure perl calculation library is the default. Hmm...