Hi,

you are reusing variable $AvailListbox. So after it gets assigned a reference to the new Listbox instance that is what it points to. You need seperate variables to hold references to different widgets. I do not see why you need the second Listbox instance at all. Why do you want an extra Toplevel for the content editing part?
As a side note: You can create scrolled widgets much easier like so: $mw->Scrolled('Listbox', @args)

Cheers, Christoph