in reply to What Perl CAN'T do?

perl cannot be written in Perl (this statement will automatically self-destruct when Perl6 comes out. Your mission, should you choose to accept it, is to assist in bringing about this event as soon as possible.)

See this thread for more answers to your question.

Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. -- Brian W. Kernighan