$hash{-10} = 'bar';
$hash{-1} = 'foo';
$hash{12} = 'baz';

%hash =
  map { (keys %hash) - 1 => delete $hash{$_} }
  sort {$b <=> $a}
  keys %hash;

print map {"$_ => $hash{$_}\n"} sort { $a <=> $b } keys %hash;