These modules are pure Perl, that's why their performance is poor Perl. (SCNR)

You might learn a bit by looking into them. :)

Good luck!

PS: I seem to remember they had XS shadows, but my brain feels 2025 years old...

Update

Yep, multiple XS modules available, like Math::BigInt::GMP and Math::BigInt::FastCalc

syphilis might know better about the pro and cons