Program that encodes a message into its own text
Perl

This is a one-liner in Perl just because it's possible.

if($ARGV[0]){open(F,__FILE__);while(&lt;F&gt;){print;print"$ARGV[0]\n"if/^_/;}}else{print&lt;DATA&gt;;}
__DATA__


The messages are written after __DATA__, most recent first.