use Tk;
$mw = MainWindow->new;
$mw->title("MainWindow");
$mw->Button(-text => "Toplevel", -command => \&do_Toplevel)->pack();

MainLoop;
sub do_Toplevel {
  if (! Exists($tl)) {
    $tl = $mw->Toplevel();
    $tl->title("Toplevel");
    $tl->Button(-text => "Close",
                -command => sub { $tl->withdraw })->pack;
  } else {
    $tl->deiconify();
    $tl->raise();
  }
}