|P is for Practical|
In my humble opinion DBI should be somewhere near the top of the list.
update But I don't think thats what you're after. I don't have any great examples of Perl being THE primary language a project was coded in. A lot of good ones have been mentioned in previous posts. I have worked with a lot of commercial software where Perl might not be what an app is primarily coded in, but has been instrumental to the point where an app might not exist were it not for Perl. Some examples avamar, vmware, and try finding a Unix/Linux/BSD distro that does not include Perl as a part of the base install.
update 2 - a question of my own - What is the length of the lengthiest node title on Perl Monks?
"That which we persist in doing becomes easier, not that the task itself has become easier, but that our ability to perform it has improved."
--Ralph Waldo Emerson
In reply to Re: What is the (greatest|biggest|largest|supreme|most known) project made (in|with) perl?