In late 80s and early 90s professional knowledge was way harder to get. Learning required significant devotion, and often was obtainable through experience only, and at the same time computer-related work wasn't as well-paid as it will become later. And had controversial social standing. Like, my brother said "It will hurt your chances with girls, bro!" when I told him I want to be a programmer, and with typical sibling-love added "This, and your ugly face of course".
RTFM emerged naturally with all of these: people paid with their time and social life for this knowledge, and wrote down what they found in manuals, most often for free, and you can't just bother to read them?
FWIW most BOFH types in my memory were C programmers, and early Linux UGs. Perlists in comparison were mild, and way more open (Perl community included biologists, linguists, and other non stereotypically computer people).
Perl decline was to some extent a cultural thing. But absolutely not the culture the author means. In Perl Larry Wall promoted a sort of expressive style of writing code where you can chose between several compiler-equivalent ways to implement logic: There is More Than One Way To Do It (aka Tim Toady) principle. The alleged reason is that the choice you make conveys some subtle nuances of your thinking which 1) gives you more comfort while coding, 2) gives potentially more information to someone who will read your code. This was outrageously contrary to the mainstream tendency of commoditization of software development (and software developers) which started to gain the steam at those times. Managers wanted (and I guess still want though to the date the idea mostly failed) THE one and only right way to code each task, peferrably with the only one language (with Java as the main contender), standard training programs, and certifications - all across all domains, and as a result replaceable and transferrable coders. Perl was clearly unfit for this perfect future with its proud selection of 4 or 5 implementations of OOP, and it hurt the language promotion a lot. And then there was disastrously optimistic declaration about soon-to-be major uprgade to Perl 6 which in reality will take 15+ years, all while lots of interesting things happening outside.