"What is the -M modifier?"

The short answer is there's no such thing (as far as I know).

In the context of this thread, '-M' is a file test operator (not a modifier). You'll also see that I've used '-f', which is another file test operator. Here's a full list of "file test operators" with an explanation of each and details of usage.

There's also the '-M' you can use when running perl. See "perlrun: Command Switches" for details.

Perhaps you were thinking of the 'm' modifier (which can be used with a regex). See "perlre: Modifiers".