use Tk;
$MW=MainWindow->new(-title=>'MyWindow');
$MW->Button(
-text=>'change',
-command=>\&change)->pack;
$LB=$MW->Label(-text=>'...')->pack;
MainLoop;
sub change
{
$LB->configure(-text=>'case 1');
sleep 3;
$LB->configure(-text=>'case 2');
}