use Tcl::Tk qw(:perlTk);
$mw = MainWindow->new;
$mw->Label(-text => "Hello world")->pack;
$btn = $mw->Button(-text => "test", -command => sub {
  $btn->configure(-text=>"[". $btn->cget('-text')."]");
})->pack;
$edt = $mw->Text()->pack;
$edt->insert('end',"\x{2456}\x{2457}");
MainLoop;