Somehow, your BigInts are transformed into double-precision floating point numbers before being handed to printf:

$ tcc -run -
#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>
int main(void) {
 printf("%" PRIx64 "\n", (uint64_t)(double)0x3243bcfe21ef4468ULL);
 return 0;
}
^D
3243bcfe21ef4400
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But you can use as_hex() of Math::BigInt (also available in new versions: to_hex()) to get the original hex representation back:

$ perl -MMath::BigInt -E'say Math::BigInt->new("0x3243bcfe21ef4468")->
+as_hex'
0x3243bcfe21ef4468
[download]