Here's an Inline::C demo that does pretty much the same thing, but bypasses the creation of a couple of extra SV's:

use strict;
use warnings;

use Inline C => Config =>
  CLEAN_AFTER_BUILD => 0,
  BUILD_NOISY => 1;

use Inline C => <<'EOC';

double wrap_cos(SV * in) {

  if(sv_isobject(in)) {
    HV *self_hv = MUTABLE_HV(SvRV(in));
    SV **callback_ptr = hv_fetchs(self_hv, "Number", 0);
    return cos(SvNV(*callback_ptr));
  }
  return cos(SvNV(in));
}

EOC

my $angle = 0.5;

my $x = {'Number' => $angle};
bless $x;

print wrap_cos($x), "\n";     # Output: 0.877582561890373
print wrap_cos($angle), "\n"; # Output: 0.877582561890373
[download]

Inline::C is just XS under the hood, and the XS file that it autogenerates and compiles (and can be found under ./_Inline/build) is:

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include "INLINE.h"

double wrap_cos(SV * in) {

  if(sv_isobject(in)) {
    HV *self_hv = MUTABLE_HV(SvRV(in));
    SV **callback_ptr = hv_fetchs(self_hv, "Number", 0);
    return cos(SvNV(*callback_ptr));
  }
  return cos(SvNV(in));
}


MODULE = try_pl_96f2  PACKAGE = main  

PROTOTYPES: DISABLE


double
wrap_cos (in)
    SV *    in
[download]

Maybe there's something there that helps you ... or maybe not ;-)
(The header file INLINE.h is generally not necessary, and is not needed here. You'll find it in the same folder as the XS file.)

Cheers,
Rob