Except maybe that's actually a thing in perl, to use this behaviour as a feature, I don't know.
Yes, that's common. List processing via variadic arity functions is an explicit feature of Perl.
Yes, that's common. List processing via variadic arity functions is an explicit feature of Perl.
sub f { return 1, 2 }
my %x = (a => f(), b => f());
I'm not sure how an appeal to PHP as lowest-common-denominator of language sanity argues against understanding the return values of functions you call.
Sure you should. But confusion between keys and values isn't useful, so there's no reason for a subexpression to be able to do that, and in PHP it can't.