For fun on the quadmath perl I use:

$ perl -V | grep nvsize
    nvsize=16
$ perl -E 'say 6**25; say ~0'
28430288029929701376
18446744073709551615
[download]

Quadmath works in this case, but of course you really want to use bigint, Math::BigInt, Math::GMP, Math::GMPz, etc. as this is just putting it off (e.g. 6^42 is exact but 6^43 is not).