There's not much to explain. The English alphabet consists of 26 letters (A through Z). You just rotate the alphabet 13 places, and now you've got N through Z followed by A through N. The most common way to do that in Perl is to use the tr/// operator to rotate the alphabet through the magic of transliteration. Try this:

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;
}
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Not exactly strong encryption. Most decent newsreaders will be able to rot-13 text. Its original use on the Net was to prevent children and employers from accidentally seeing a curse word in some Usenet post. But the concept has been around since 3rd grade 200 years ago when Cracker Jacks first started putting decoder rings in their popcorn boxes. ;)