for HList and the Tree derivative you can't use the yviewScroll(...) method to mean yview('scroll', ...)
You must use the yview('scroll', ...) form:

    $tree->bind("<Button-4>", sub { $tree->yview('scroll',1,"units") }
+);
    $tree->bind("<Button-5>", sub { $tree->yview('scroll',-1,"units") 
+});
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