Hi Hauke, I and countless others have written at great length here and all over the place about the actual real-life employment/business effects of the years of Raku interlopers using Perl's name, resources and culture to market itself and muddy the waters for Perl itself. It's a very old, very well-documented story and I'd be surprised if you weren't aware of it.
Update: Here's the most recent article on the topic, by ovid.
As for your question about source, I am weary of rehashing this debate that has already been settled (except, apparently, for here), but I respect you greatly and will point out that I have already posted links and quotes from the Raku discussion that you originally linked to, where Liz M clearly states that the "sister languages" meme was damaging to both languages. She used the word "meme." The admission of damaging effects was new, but she admitted two or three years ago in a Reddit discussion that the "sister language" narrative was a deliberate fiction created to try to ameliorate the fact (which had become apparent to the world by then) that Rake was *not* a new version of Perl. She, mst, jberger and many others have written about the origin of that.
There are thousands of working professional Perl programmers, including me, who have had to live with the confusion that a handful of bored brainy people have sowed, under the blessing of Larry Wall, for almost two decades. I work in a large corporation -- > 3,000 people -- and Perl powers all our core APIs, yet even within the company there's the "Perl is old, good for scripting" perception among managers and IT people who only know what flashes across the Reddit.
People who say Raku is Perl do not make their living as Perl programmers.So yes, it's personal, besides the moral imperative to challenge lies and the ugliness of a cukoo stashing its egg in another bird's nest so it will raise it.
The way forward always starts with a minimal test.