Anything done all the time "because someone said so" is "cargo cult".

However, just because it is "cargo cult" doesn't mean it can't be a best practice. So the question more usefully becomes "Is always using xms on regexen a good idea?".

Personally I can see the justification for generally using /x and /s - they tend to avoid surprises, like when a chunk of whitespace is a tab rather than the space expected and treating all characters in a non-special way (/s) saves involving a couple of brain cells during processing more often than it requires them.

But /m is a beast of a different stripe requiring a significant switch of meaning for ^ and $. I haven't read the book (books are expensive here and Perl is something of a "sideline" for me) so I don't know the justification, but I'd be inclined to leave the /m switch off.