use Tk;                                                                                                         
                                                                                                                
$main = MainWindow->new();                                                                                      
                                                                                                               
$button = $main->Button(-text => "step one", -command => \&fun);
                                                                                                                
$button->pack();                                                                                               
                                                                                                                
MainLoop();                                                                                                     
                                                                                                                
sub fun {
 $button->configure(-text => "step two" );
 sleep 1;
 $button->configure(-text => "step three" );
}