#!/usr/bin/perl

# http://perlmonks.org/?node_id=1166828

use strict;
use warnings;
use Tk;

my $alt = 0;

my $mw = new MainWindow;
my $e = $mw->Entry(-show => '*',
  )->pack();
$mw->Button(-text => 'Toggle',
  -command => sub { $e->configure( -show => $alt++ % 2 ? '*' : '' ) },
  )->pack();

MainLoop;
[download]