string.c:1743
static VALUE
uscore_get()
{
    VALUE line;

    line = rb_lastline_get();
    if (TYPE(line) != T_STRING) {
	rb_raise(rb_eTypeError, "$_ value need to be String (%s given)",
		 NIL_P(line) ? "nil" : rb_obj_classname(line));
    }
    return line;
}
