$ perl -MB=svref_2object,SVf_IVisUV,SVf_NOK -e'
   $i = 0;
   $sv = svref_2object(\$i);
   print $sv->FLAGS & SVf_NOK    ? "NV\n"   # Float
      :  $sv->FLAGS & SVf_IVisUV ? "UV\n"   # Unsigned int
      :                            "IV\n";  # Signed int
'
IV