use Tk;
$mw = MainWindow->new;

$e1 = $mw->Entry->pack;
$e2 = $mw->Entry->pack;
$sum = $mw->Label->pack;

$mw->Button(-text => 'Compute', -command => sub {
        $sum->configure(-text => ($e1->get() + $e2->get()));
})->pack;

MainLoop;