The trouble with MLDBM

First of all, there is an error in the text on page 243 of the text. It is in regard to MLBDM. The correct code is
my $mary = $hash{mary};
$mary->{email} = 'mary@here';
$hash{mary} = $mary;
The thing that is retrieved from the database is an object, not a hash. It is necessary to treat it as an object by using the -> operator to reference a field.

The above is the method for modifying a record that is already in the database.

It is not possible to edit the database directly with a statement like
$hash{mary}{email} = 'mary@here';

Furthermore, it is not possible to change the entire record to a hash with a statement like
$hash{mary} = %some_hash;

You will need to use a reference to do this
$hash{mary} = \%some_hash;

If you have done the above and stored the record as a hash, then, to modify the data in the record, first copy it back to a hash
%some_hash = %{$hash{mary}};
$some_hash{'city'} = 'bedrock';
$some_hash{'pet'} = 'dino';
$hash{mary} = \%some_hash;