warn("ST(0) is of type %s\n", HvNAME(SvSTASH(SvRV(ST(0)))));

##</code><code>##

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

use Inline C => <<'EOC';

void foo(SV * x) {
    dXSARGS;
    if (sv_isobject(ST(0)))
        warn("ST(0) is of type %s\n", HvNAME(SvSTASH(SvRV(ST(0)))));
    else warn("Not an object\n");
    XSRETURN(0);
}


EOC

use Math::BigInt;
my $obj = Math::BigInt->new(17);
foo($obj);