$ perl -MConfig -MB=svref_2object,SVf_IVisUV,SVf_NOK -e'
   $i = eval($Config{nv_overflows_integers_at}) - 3;
   $sv = svref_2object(\$i);
   for (1..4) {
      ++$i;
      printf $sv->FLAGS & SVf_NOK    ? "NV %.0f\n"
         :   $sv->FLAGS & SVf_IVisUV ? "UV %u\n"
         :                             "IV %d\n", $i;
   }
'
NV 9007199254740990
NV 9007199254740991                 Precision required as a float:
NV 9007199254740992  <-- 2**53      1 bit
NV 9007199254740992  <-- 2**53 + 1  54 bits, but only 53 are available