If you run this in a loop, you'll eventually get to a point where all the values are non-keys. But this solution assumes you are not interested in the recursion. It also assumes you want to process every key in the hash and not just "0".

  my %newhash;
  foreach my $key ( keys( %hash ) ) {
    $newhash{$key} = [ map {exists($hash{$_}) ? $hash{$_} : $_;} (valu
+es %hash) ];
  }
  %hash = %newhash;
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