$mw -> Checkbutton (
           -activebackground => "white",
           -command => &react ("\"$x\_$y\"");
           -command => print "hi"  # <= this is a change
           ) -> grid (
                -row => $x, 
                -column => $y
                );
....

##</code><code>##

           -command => sub {print "hi"}  # <= this is a change
....