If you care about performance, the following avoids some hash lookups and so should perform better: 
sub invert {
  my $x = shift;
  my $y = {};
  while (my ($key1, $value1) = each %$x) {
    while (my ($key2, $value2) = each %$value1) {
      $y->{$key2}{$key1} = $value2;
    }
  }
  return $y;
}

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I don't think you can get much faster than this in pure Perl.

If you want to speed this up, save memory, etc, then you have to look into why you find the need to invert multi-level hashes regularly. Since you haven't shown an example of a design problem where you find yourself wanting to do that, I can't comment on whether your design is appropriate or whether there is an alternate way to tackle those problems that would work better.

In short I can think of some situations where inverting a hash is absolutely the right thing to do. I can think of others where there is no real need to do it. Without more context, there is no point in my guessing which applies to you.

In reply to Re: Invert a hash... not a FAQ (I hope) by tilly
in thread Invert a hash... not a FAQ (I hope) by djacobow

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