Another way to do it is to use the -textvariable attribute of the Button widget. What that does is bind the button text to a variable and whenever the variable changes, the button text will automatically change.

A modification of the code above:

#!/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';
}
[download]

The benefit of doing it this way is that you don't have to worry about explicitly updating the widget yourself. It happens automatically whenever the value of the bounded variable changes.

hope this helps,
davidj