$ perl -MConfig -MB=svref_2object,SVf_IVisUV,SVf_NOK -e'
   $i = hex(("FF"x($Config{ivsize}-1))."FD");
   $sv = svref_2object(\$i);
   for (1..4) {
      ++$i;
      printf $sv->FLAGS & SVf_NOK    ? "NV %.0f\n"
         :   $sv->FLAGS & SVf_IVisUV ? "UV %u\n"
         :                             "IV %d\n", $i;
   }
'
UV 18446744073709551614
UV 18446744073709551615  <-- 2**64 - 1  Largest UV
NV 18446744073709551616  <-- 2**64      1 bit of precision required
NV 18446744073709551616  <-- 2**64 + 1  65, but only 53 are available