You could do it like this:

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');
}
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