I was only allowed to use perl 4 to write this software and I wasn't allowed to use a database, even though the datastructure for a day's publishing batch had tens of thousands of keys and values and required RDBMs style queries. It also features a configurable postscript parser that extracts all kinds of data from completed newspaper pages that informs the publishing system. When I wrote it I was told that it would run for a few months only while we figured out how to get a $5M commercial product to handle the work.
The whole thing was written in perl 4 style OO Perl and came in at about 16k lines of code in the end -- most of the code was for the postscript processor and tons of cruft that I had to write to made a relational DBMS in memory because I wasn't allowed to use mysql. It took me four months to write it. I launched it in January of 2002 and it runs to this day. I know this because I got a call about it last month where my replacement's replacement's replacement asked me a few questions about what OO Perl is because he wanted to make a few changes. Good luck! It still runs and is responsbile for about 80% of what it was originally built to do. It is used by hundreds of people daily, who by all reports absolutely hate it. There are people working at the newspaper today that use it regularly that were not born when I wrote it. I am twice as old as I was when I wrote it.
They have apparently tried to replace it several times over the last 22 years and have failed to do so... this is likely due to the blockheadedness of my old boss (who is still there) as much as the radically insane obscurity of my code, which is exactly how you'd expect 16k lines of 22 year old OO Perl 4 would be.