use strict;
use warnings;
use Inline 'C';

test_err();

__END__
__C__
#define xstr(s) str(s)
#define str(s) #s
void test_err() {
    (void)fprintf(stderr, "ERROR goes to " xstr(stderr) "\n");
}

# here, it outputs:
# ERROR goes to (*(*Perl_IStdIO_ptr(((PerlInterpreter *)Perl_get_context())))->pStderr)((*Perl_IStdIO_ptr(((PerlInterpreter *)Perl_get_context()))))