For reference:

Valid is either

   sub () {
     my ($x, $y, $z) = @_;

or

   sub ($x, $y, $z) {

I should have said

  my ($x, $y, $z) = @$_[0];

but I was making a joke.

If $_[0] is an array reference, @$_[0] becomes a list.

The joke was a reference to most wrong Perl programs being valid for something.

Hence the meme: "93% of Paint Splatters are Valid Perl Programs"

https://www.mcmillen.dev/sigbovik/

Why the empty prototype in the first alternative?

yeah, you could leave that out, but then I have to invent a name

Really? Anonymous subroutines/coderefs are defined with sub {} are they not?