Still, I do wonder if defaulting to /msx is concensus or the opinion of a few. I've certainly never encountered many people expressing this opinion before PBP came out.
Like many good--and bad!--memes, it certainly seems to have started with one person (i.e. me). Like you, I had never seen anyone else suggest permanent /xms
until I started using it.
But I do think that the idea has spread rapidly and is become a consensus best practice, not simply because it's Damian who advocates it, but because many people are finding it valuable, finding that it improves their coding and simplifies their maintenance.
Of course, that's not to say that there aren't some people who do just blind adopt it because I said so. I'm sure there are such cases. Unfortuately, there's nothing I can do about that. I spent most of the first chapter of PBP pleading for readers to assess, consider, try, and then critically evaluate my advice, but not everybody has the will, the interest, the time, or (frankly) the capacity to do that. For those that don't, at least they're blindly adopting a convention that works, that is consistent, that is less error-prone than the Perl 5 defaults, that has few drawbacks or downsides, and that many more clueful Perl folks agree is a good idea and have adopted themselves.
I fully accept that other clueful Perl folks, such as yourself, don't like the convention, and find it annoyingly constraining or less expressive or fun-diminishing. However, I'd point out that in many development environments there are good and valid reasons to restrict the freedom, expressiveness, and playfulness of individual developers, not the least of which is that not all developers in the environment are sufficiently flexible, literate, or self-disciplined to cope with the freedom, expressiveness, and playfulness of their more accomplished colleagues.