If I want to know whether a value is an integer value (IV) or not (as often happens), I just use the the XS function SvIOK().
If it returns true then it's an IV, otherwise it's something else.

#!perl -l
use strict;
use warnings;
use Math::BigInt;


use Inline C => Config =>
 BUILD_NOISY => 1;

use Inline C => <<'EOC';

int is_IV(SV * x) {
  if(SvIOK(x)) return 1;
  return 0;
}

EOC

my $x = '42';
print is_IV($x);   # 0 

$x += 0;
print is_IV($x);   # 1

print is_IV(~0);   # 1 (also UV)
print is_IV('?');  # 0;

my $mbi = Math::BigInt->new(1);
print is_IV($mbi); # 0;

print is_IV(2 ** 20); # 0 (NV)
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Cheers,
Rob