At the very least, I've seen Data::Dumper get painfully slow with complex data structures. It might be easier and faster to simply roll your own. Something like this would probably get you 90% of the way there:

foreach my $key1 (keys %hash) {
  print "$key1:\n";
  foreach my $key2 (keys %{$hash{$key1}}) {
    print "  $key2:\n";
    foreach my $key3 (keys %{$hash{$key1}{$key2}}) {
      print "    $key3:\n      " , 
            join (",",@{$hash{$key1}{$key2}{$key3}}), 
            "\n";
    }
  }
}
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        $perlmonks{seattlejohn} = 'John Clyman';