use Math::BigInt lib => 'GMP';

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


use Inline C => <<'EOC';

#include <gmp.h>

mpz_t * mpz_from_sv_nofail (SV *sv)
{
  MAGIC *mg;

  if (!sv_derived_from(sv, "Math::BigInt::GMP"))
    croak("not of type Math::BigInt::GMP");

  mg = SvMAGIC(SvRV(sv));

  return (mpz_t *)mg->mg_ptr;
}

mpz_t * mpz_from_sv (SV *sv)
{
  mpz_t *mpz;

  if (!(mpz = mpz_from_sv_nofail(sv)))
    croak("failed to fetch mpz pointer");

  return mpz;
}

void my_mul(SV * bi_rop, SV * bi_op, SV * si) {
     mpz_t *t1, *t2;

     t1 = mpz_from_sv(bi_rop);
     t2 = mpz_from_sv(bi_op);

     mpz_mul_si(*t1, *t2, SvIV(si) * 10);
}

EOC

my $rop = Math::BigInt->new(0);
my $op  = Math::BigInt->new(123456);
my $x   = -110;

my_mul($rop->{value}, $op->{value}, $x);
print $rop, "\n";

__END__
Outputs:
-13580160