Without an explicit return, a subroutine returns the final statement evaluated, which in your case is

   @_
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If you defined do_nothing as:

sub do_nothing {}
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Then you'd get a different result, namely there'd be nothing in @out

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Update: Sorry, didnt test this. You will actually get the error:

Sort subroutine didn't return a numeric value at nothing.pl line X

if you define do_nothing() as i described. But the reasoning still stands.