64-bit Perl is as stable as your platform/compiler is. But 64-bit integers means at that you are limited to the range -18446744073709551616 to 18446744073709551615, while Math::BigInt gives you virtually unlimited sized integers (you're only bound by the size of your virtual memory). Of course, 64 bit integers, whether native or simulated, are much, much, much faster than Math::BigInt integers.

Abigail