"it almost had a "first mover" advantage in that it happened to be installed alongside CGI-BIN on shared webhosts way back in the way"
That's a good observation. Because the mod_php and mod_perl Apache modules were much faster and lighter weight than CGI-BIN, and pre-installed on shared hosting, both PHP and Perl dominated the early web days.
When / where was mod_perl ever widespread? Perl was usually provided via CGI, and php was mostly deployed with mod_php back in the day which gave it a great speed advantage, often at the price of security.
I know that Perl was earlier, I've worked with Perl before PHP came around. However, I've never seen a shared hosting environment that offered mod_perl, they were all using CGI.
Might be region specific, I've mostly worked in Western Europe, but even the two NA-based providers I've used back then didn't offer mod_perl for shared hosting.
You could certainly get it on a managed server, or set it up yourself on your dedicated box, but PHP's strong suit was the low-end shared hosting options. It ran anywhere, and usually fast.
That's a good observation. Because the mod_php and mod_perl Apache modules were much faster and lighter weight than CGI-BIN, and pre-installed on shared hosting, both PHP and Perl dominated the early web days.