I don't know what the original service was written in but I think a rewrite started in 2007:
https://web.archive.org/web/20070811100100/http://www.oclc.org/news/releases/200669.htm
The federated approach and/or the software failed and it moved to the Internet Archive in 2016?
https://blog.archive.org/2016/09/27/persistent-url-service-purl-org-now-run-by-the-internet-archive/
I'm not sure if they are running the same service or if it is something new.
I think some people got frustrated and created https://w3id.org/ which is basically some Apache rewrite rules maintained with Git.
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