my $string = "Cleartext.";
print $string, "\n";
$string = rot13( $string );
print $string, "\n";
$string = rot13( $string );
print $string, "\n";

sub rot13 {
    my $text = shift;
    $text = tr/A-Za-z/N-ZA-Mn-za-m/;
    return $text;
}