my $scrollbar = $mw->Scrollbar( );
my $lb = $mw->Listbox(-yscrollcommand => ['set' => $scrollbar]);
#Configure the Scrollbar to talk to the Listbox widget
$scrollbar->configure(-command => ['yview' => $lb]);

#Pack the Scrollbar first so that it doesn't disappear when we resize
$scrollbar->pack(-side => 'right', -fill => 'y');
$lb->pack(-side => 'left', -fill => 'both');