BigInt / pack problems

[Shmuel Fomberg]

Hi All.

Can someone with tell me why this code:
(you will need Perl 5.10 and 64 bit support and Math::BigIn installed)

print unpack "H*", pack "Q>", Math::BigInt->new("18446744073709551360");

prints 'ffffffffffffffff' instead of 'ffffffffffffff00' that I expect it to?

Thanks,
Shmuel.