#!/usr/bin/perl

use strict;
use Tk;

my $FILTER_IS_ON = 0;
my $tv = $FILTER_IS_ON ? 'turn off' : 'turn on';

my $top = new MainWindow();

my $b = $top->Button(-textvariable=> \$tv,
                     -command => \&turn)->pack();

MainLoop();

sub turn {
    $FILTER_IS_ON = $FILTER_IS_ON ? 0 : 1;
    $tv = $FILTER_IS_ON ? 'turn off' : 'turn on';
}