As an additional datapoint, if you call it with a full complement of parentheses:

my @out = sort(do_nothing(@in));
[download]

the output is sorted, because sort knows that it has only one argument. Sort is definitely thinking that do_nothing is its comparator, and is putting it in scalar context. Here's an interesting variation:

sub do_nothing { wantarray ? @_ : -1 }
[download]

yields scrambled output, because it's a faulty comparison function. So if you make it

sub do_nothing { wantarray ? @_ : ($b cmp $a) }
[download]

you get reverse-sorted output. But I don't know the innards of perl well enough to say why it is parsing it that way, though I can kind of understand what's happening: you can say sort numerically @array and it recognizes that numerically is a function. And (@array) is really the same as @array, so that understanding still works. If you provide the function-calling sigil, it gets a clue:

my @out = sort &do_nothing(@in);
[download]

yields sorted output.