hex can only handle numbers as big as the integers your Perl can handle. On a Perl with 32-bit integers,

>perl -le"hex('0xFFFFFFFF')"

>perl -le"hex('0x100000000')"
Integer overflow in hexadecimal number at -e line 1.
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Math::BigInt's from_hex could be of use. There could be other solutions as well if we were familiar with the bigger picture.