Caeser Shift encryption is one of the simplest. You simply think of a number, and move the letters of your message along that amount of letters (looping to 'a' after a 'z').
Thus, this message:
veni, vidi, vici
after a shift of 5 becomes:
ajsn, anin, anhn
The perl script here (rename it to .pl instead of .txt) simply swaps the letters along n times, where n is a commandline argument.
Place your message in a file called plaintext.txt
./caesershift.pl 5
Change the '5' to whatever you want. The encrypted message is written to a file called encrypted.txt.
See here for a script which decrypts caeser shift messages.