Back in the day pl was a real problem.

It was and is the conventional extension for the Perl programming language, and Perl was most popular for server side (CGI) scripts for some time.

At same time the idea that webserver and user agent should negotiate the most suitable form of content was all the rage. Little of that survived (compression algorithm) but it used to be used for language as well.

Apache had the convention to use the two letter language code as file extension to encode the language. For example it shipped with collection of error messages in many languages, e.g. 404.html.pl.

Since this conflicted with Perl CGI scripts, Perl got .plx for Perl Executable in Apache and it was proposed as the official Perl extension, but did not stick.