Both concise and weird. And no, it's not really golfed. I just wanted to see if this would work.

sub two_sum # O(n)
  {
  my ($i, $input, $target, %h) = (0, @_);
  return [ $h{$_} // ($h{$target - $_} = $i++, next), $i ] for @$input
+;
  }
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