You want to use the perlguts macros SvIOKp, SvNOKp, SvPOKp (test for internal representation as IV (integer), NV (double) or PV (string)). Here's Inline code to do it (you'll need to get the Inline module from CPAN):

package types;

use Inline C => <<'END_C';

int is_iv(SV *x) {
  return SvIOKp(x) != 0;
}

int is_nv(SV *x) {
  return SvNOKp(x) != 0;
}

int is_pv(SV *x) {
  return SvPOKp(x) != 0;
}

END_C

1;
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But I'm not sure I'd recommend doing this! Perl users expect to have all integers autostringified when needed. Also, I wouldn't try it on magic "double valued scalars" (which have different string and numeric values), like $!.