I don't get that behaviour on the 64-bit 5.8.8 Perl (little-endian system) to which I have access. 
$ perl -MMath::BigInt -le'print unpack "H*", pack "Q", Math::BigInt->n
+ew("18446744073709551360");'
00ffffffffffffff

$ perl -MMath::BigInt -le'print unpack "H*", "".reverse pack "Q", Math
+::BigInt->new("18446744073709551360");'
ffffffffffffff00

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In reply to Re: 64 bit integers woes by ikegami
in thread 64 bit integers woes by semuel

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