if you're using /s and you want the "match anything except a newline" behavior, does not [^\n] best express this?

In the context of /s, sure. But again, there is a larger context: someone else reading the regex will wonder at the incongruity of using /s at all in conjunction with a bunch of [^\n]s, when omitting the /s would simplify the whole thing (and also make the regex syntax match that used by most other regex engines, a consideration — albeit minor — when aiming for clarity).

One of the driving motivations of TheDamian's regex PBPs is clarity of expression.

A goal we can all agree with! Now, enter devils and details.

Anyway, TheDamian explains it all more completely and clearly than I can, so maybe pursue that avenue.

Point taken. I'm sure my perl experience is a tiny fraction of either yours or his.