Pure-perl alternatives for XSish modules are always useful, for people stuck on Windows or Mac or similarly compiler-less systems. As for perl6 - it doesn't exist yet, and isn't expected any time soon. Don't wait to release stuff just because we'll get it in perl6.

However, check to see that Math::Bigint doesn't already do what you want. It's part of the core in 5.8, and possibly in earlier versions, I disremember.