my %hash = tie( ('a' => 1), 'IxHash' );

##</code><code>##

   my %big_hash = (
       'part_1' => tie( %{...}, 'MyTie'),
       'part_2' => tie( %{...}, 'MyTie'),
       ...
   );

##</code><code>##

   tie my %part_1, 'IxHash';
   # set values for %part_1
   tie my %part_2, 'IxHash';
   # set values for %part_2
   # ...

   my %big_hash = (
      'part_1' => \%part_1,
      'part_2' => \%part_2,
      ...
   );