++Anonymous Monk ... One important thing to note about tied hashes with MLDBM though is that the value of a hash should be copied to a temporary variable prior to its manipulation. From the BUGS section of the module POD ...

$mldb{key}{subkey}[3] = 'stuff';        # won't work

tmp = $mldb{key};                       # retrieve value
$tmp->{subkey}[3] = 'stuff';
$mldb{key} = $tmp;                      # store value
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This limitation exists as a result of the Perl TIEHASH interface having no support for multidimensional ties. I wrote about this topic in a more general way in The importance of context in debugging.