I never used Tk::Listbox so this is little more than a guess but, based on a cursory read of POD, I think that the methods you need are listbox->activate($index) that makes the element of index $index the current element and listbox->selectionSet($index,$index) that put the element $index in the selection set.

Obviously you need to know the index of the element you want to activate, so you'll probably have to search for it in the listbox itself.