use Math::GMP;
use Math::GMPz;

use Inline C => Config =>
    LIBS => '-lgmp',
    BUILD_NOISY => 1,
    USING => ParseRecDescent;

use Inline C => <<'EOC';

#include <gmp.h>

void my_mul_si(SV * rop, SV * op, SV * si) {
     mpz_mul_si
     (*(INT2PTR(mpz_t *, SvIV(SvRV(rop)))),
      *(INT2PTR(mpz_t *, SvIV(SvRV(op)))),
      SvIV(si));
}

EOC

my $ x = -10;

my $g_rop = Math::GMP->new();
my $g_op  = Math::GMP->new(1234567);

my $z_rop = Math::GMPz->new();
my $z_op  = Math::GMPz->new(1234567);

my_mul_si($g_rop, $g_op, $x);
my_mul_si($z_rop, $z_op, $x);

my_mul_si($g_op, $g_op, $x);
my_mul_si($z_op, $z_op, $x);

print $g_rop, "\n", $z_rop, "\n",
      $g_op,  "\n", $z_op, "\n";

__END__
Outputs:
-12345670
-12345670
-12345670
-12345670