Use Math::BigInt to let them grow unboundedly. (This module underwent a major rewrite at one point. Use the version shipped with perl 5.8 or later; for earlier perl versions, get a copy from CPAN).

Normally, perl uses a 32 or 64 bit signed integer (depending on platform and Configure options), promoting it to a floating point value (usually with 53 or 64 bits of mantissa)/ as needed. To stretch just a little extra range for integers, perl will sometimes use an unsigned integer instead of a signed one.