use Tk;
$mw = MainWindow->new;
$mw->title('Sample');

$e1 = $mw->Entry(-textvariable => \$var1)->pack();
$e2 = $mw->Entry(-textvariable => \$var2)->pack();
$e3 = $mw->Entry(-textvariable => \$var3)->pack();
$b = $mw->Button(-command => sub { $var3 = $var1 + $var2; })->pack();

MainLoop;