use Tk;
use warnings;
use strict;

my $count = 0;
my $mw = MainWindow -> new;
$mw -> Label(-text => 'Counter') ->pack();
$mw -> Label(-text => $count ) ->pack();
$mw -> Button( -text    => 'Count', -command => sub { $count++; $mw -> update; } ) ->pack();

MainLoop;