Well you can get to the actual scrollbar with the Subwidget method.

my $scrollery = $plist ->Subwidget("yscrollbar");

$scrollery ->configure(-background => "green",
              -troughcolor => "black",
              -command => \&somecallback );
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Now whenever the y scrollbar is moved, the callback will be executed. Read "perldoc Tk::Scrollbar"