Unix Perl - Class Mission 
6/5 

You are to implement the incredibly secure ROT-13 encryption system using the immensely useful (and wonderfully cross-platform) Perl language. Headquarters has chosen this method to test your fledgling Perl skills because the simple algorithm leaves more time for you to work on bettering your application of it. Its text-processing power is perfectly suited for such a task, and its availablity on a variety of platforms means that your solution will work for a variety of our financially interested partners.

Here is a list of web contacts that you may find useful:
The secluded Perl Monks may be able to aid you with vital information.
The Comprehensive Perl Archive Network almost certainly encompasses something of value.
Use Perl has been a trusted source in the past, but you may need some backup before entering the more populated areas.
O'Reilly has released The Llama Book which has made available online through a secret partnership with UW libraries.

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The Details

It works like this:
The cleartext is converted to encrypted form by changing every letter to the one 13 later than it in the alphabet. For the purpose of the assignment you are to assume that the alphabet is circular, rather than linear like you were taught in primary school. It's like using a decoder-ring from the cereal box (see the picture to the right to get some idea of what I mean if your cereal never came with prizes). So the song would go like "...xyzabcdefg..." Repeating the process recovers the original text, because each letter is rotated around to its original value.

Because the rot13 encryption standard originated in the hard world of Usenet, a variety of other solutions exist for comparison. Headquarters insists on a fresh implementation, but the Jargon File has details that you may find historically interesting.
Example

To be sure that the assignment is clear and to test your possible solutions, the following example has been prepared:

Enter your text, then click the button to (de/en)crypt it.