I suppose one could use sub f { return (); } if (f()) {}, but that's a very bad practice. (A sub that's expected to return a scalar should do so even in list context.)

I agree that subs that are expected to return a scalar should do return undef because of the problems with my %hash = ( k => f() );, but there are also some good arguments against return undef in other cases - as you've said, there are exceptions.

I mostly mentioned it because the old section "Truth and Falsehood" mentioned it, although with an attempt to improve the wording, as you've suggested yourself in the past.

Update: I missed your ninja edits, but I hope I've cleared up your bafflement nonetheless. Please feel free to add any information you think is missing!

Logically, operators like grep, map, m//, and s/// all return false by returning an empty list, at least in a simple description.

That makes no sense. An empty list isn't false. Or true. It's not a scalar, so those concepts don't apply to it. You can't evaluate whether an empty list is true or false. You can only evaluate if a scalar is true or false.

As such, if you're trying to get a true or false value from those operators, you would be evaluating them in scalar context, and the following is what the listed operators actually return in scalar context:

• grep returns the count of matching elements.
• map returns the count of elements it would return.
• m// and s/// return success or not.