By now, we have all seen self modifying code. So, I won't bore you with something simple as that. Instead, I will present you with self correcting code. The program below does not compile. However, if you save the code into a file, (make sure you do not put a trailing newline in the file), and execute the file, it will print the famous message. When the process is finished, you are left with code that compiles - and executing the file again will not print anything.

#!/opt/perl/bin/perl   --   # Remove trailing newline!
BEGIN{$SIG{__WARN__}=sub{$_=pop;y-_- -;print/".*(.)"/;
truncate$0,-1+-s$0;exec$0;}}//rekcaH_lreP_rehtona_tsuJ
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-- Abigail