I suspect they're pointing out the fact that the module name is Tk::HListplus and not as you have in your code HListplus dispite your claims to have copied the code from the documentation.

Ok, so Tk::HListplus, it says it is derived from HList, that must mean Tk::HList, so if you go to Tk::HList you can see documentation for ->add and ->addchild

->add has interesting documentation about options -at -after and -before