#!perl 
use warnings;
use strict;
use Tk;

my $mw = MainWindow->new;
$mw->title("Checkbutton Grid");

my $x_size = 5;
my $y_size = 5;
my %b_name;

for (my $x=0; $x<$x_size; ++$x) {
  for (my $y=0; $y<$y_size; ++$y) {
    $mw -> Checkbutton (
           -activebackground => "white",
           ) -> grid (
                -row => $x, 
                -column => $y
                );
  }
}
MainLoop;

##</code><code>##

    $mw -> Checkbutton (
           -activebackground => "white",
           -command => &react ("\"$x\_$y\"");
           -command => print "hi"  # <= this is a change
           ) -> grid (
                -row => $x, 
                -column => $y
                );
....
sub react {
  print "$_[0]\n";
}

##</code><code>##

-command => [\&display, $x, $y]