I just had to check if it even existed because I was sure that I had a CGI book that focused on Perl from O'Reilly in the late 90's, and sure enough, the book I had was published in 1996 (with a second edition released in 2000).

Not saying your anecdote is inaccurate, but my perception around that time was that "Learn PHP in 24 Hours" was a lot hotter than O'Reilly's Perl books - so it may have just been luck, marketing, a flashier title, or even just that PHP was better suited for what people wanted to learn and do.