Just unpack a signed long long int instead and then convert it to unsigned by bitwise anding with 0xffffffffffffffff (with the right number of fs in case I goofed it up). Or try Math::Int64::native_to_uint64.

Update: If you are on a 64-bit platform (or you are on a 32-bit platform but your perl is specifically compiled so that perl integers are 64 bit which is rare), then you can use the "J" unpack template, and, in the first case, "L!" as well. Otherwise you could also unpack two 32-bit integers and combine them in the right direction.