This might work:

use strict;
use warnings;
use Tk;
use Tk::Adjuster;

my $t = MainWindow->new;

my $w1   = $t->Listbox(-background => 'yellow', -height => 3,);
my $adj1 = $t->Adjuster();
my $w2   = $t->Listbox(-background => 'blue');

$w1->pack(-side => 'bottom', -fill => 'both', -expand => 1);
$adj1->packAfter($w1, -side => 'bottom');
$w2->pack(-side => 'bottom', -fill => 'both', -expand => 1);

MainLoop;
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