In the example below, I would like to keep the bindings provides by the Scrolled widget - when user clicks or or drags the scrollbar elements - but I'd like to be called so that I can add some behavior. Specifically, when user scrolls down to the last planet, I'd like to add to the list some exoplanets, for example. When called, I would also need to know/find out what is the current scroll position. Is there a general pattern for doing this in Tk? Pointers to doc, tutorials or working examples would be welcome.

#perl -w

use strict;
use Tk;

my @planets = qw / Mercury Venus Earth Mars Jupiter Saturn Uranus Nept
+un Pluto /;

my $mw = MainWindow->new();

my $plist = $mw->Scrolled(
    "Listbox",
    -scrollbars => 'oe',
    -selectmode => 'single',
    -height     => 4,
    -setgrid    => 1
);

foreach my $planet (@planets) {
    $plist->insert( 'end', $planet );
}

$plist->pack();

MainLoop();
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Rudif