As an additional datapoint, if you call it with a full complement of parentheses: 
my @out = sort(do_nothing(@in));

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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 }

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yields scrambled output, because it's a faulty comparison function. So if you make it 
sub do_nothing { wantarray ? @_ : ($b cmp $a) }

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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);

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yields sorted output.
Caution: Contents may have been coded under pressure.

In reply to Re: Perversity of sorts by Roy Johnson
in thread Perversity of sorts by tlm

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