I have a complex data structure, an example of which can be created using the following code:

use Data::Dumper;

my $free_table = {};
my $mastertitle = "title";
my $elementid = "foo";
my @unique_terms = qw(These guides were given a plug a few months ago 
+before the JISC conference in fact, where they were displayed and I f
+orget to pick them up. However, I rectified that oversight on Friday 
+at that aforementioned news working group in the JISC offices in cent
+ral London.);

foreach my $index_term (@unique_terms) {

  # we have a three-level checking process to deal with
  
  # The word itself
  $free_table->{$index_term} = {}
    unless exists $free_table->{$index_term};
  print "term = $index_term\n";
  
  # The specific institution subdivision
  $free_table->{$index_term}->{$mastertitle} = {}
    unless exists $free_table->{$index_term}->{$mastertitle};
  print "mastertitle = $mastertitle\n";

  # "Archdesc" isad or "fond" isad
  # make a reference to the anonymous hash at this bottom level
  $data_type = int (rand 2) + 1;

  # Add the element to the bottom level
  $free_table->{$index_term}->{$mastertitle}->{$data_type} = {}
    unless
      exists $free_table->{$index_term}->{$mastertitle}->{$data_type};
  
  $free_table->{$index_term}->{$mastertitle}->{$data_type}->{$elementi
+d} =
    1;  
}
print( Dumper ( %{$free_table} ) );
[download]

Now - this is fine, and everything works as expected. If I now try to extend the concept so that the data structure is help on disk, I'd do something akin to:

unless ( -r "$repository_root/free/freetext.mldbm" )
       { %free_table = (); }
tie %free_table, 'MLDBM', "$repository_root/free/freetext.mldbm",
  O_CREAT | O_RDWR, 0640
  or die $!;

$free_table = \%free_table;
&build_data_structure_as_above(@some_params);
print( Dumper ( %free_table} );
[download]

However this only gives me a %free_table as an empty hash. If I change my code so that I'm not writing to a tied hash, things are different:

unless ( -r "$repository_root/free/freetext.mldbm" )
       { %free_table = (); }
tie %free_table, 'MLDBM', "$repository_root/free/freetext.mldbm",
  O_CREAT | O_RDWR, 0640
  or die $!;

$free_table = {};
&build_data_structure_as_above(@some_params);
%free_table = %{$free_table}
print( Dumper ( %free_table} );
[download]

This works.. The question is ... why?