Beginner TK question:

If a button label changes, how does one ask the widget to refresh itself? Thanks. 

my $on_off = $left_frame->Button(
    -text => $FILTER_IS_ON ? 'turn off ' : 'turn on',
    -command => sub {
        $FILTER_IS_ON = $FILTER_IS_ON ? 0 : 1;
    },
    -anchor => 'w',
)->pack( side => 'left' );

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In reply to TK: button update by water

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