use Tk;

my $FILTER_IS_ON = 0;

my $top = new MainWindow();

my $b = $top->Button(-text=>$FILTER_IS_ON ? 'turn off' : 'turn on',
		-command => \&turn)->pack();

MainLoop();

sub turn {
	$FILTER_IS_ON = $FILTER_IS_ON ? 0 : 1;
	$b->configure(-text=> $FILTER_IS_ON ? 'turn off' : 'turn on');
}