Note: I don't know anything about NDMB_File, so I'm working with the structure you showed.

For starters, Data::Dump is your debugging friend here:

use Data::Dumper;
print(Dumper(\%db));
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To move around your structure, have a look at these two snippets:

foreach $key (keys(%db)) {
   for ($record_num=0; $record_num<@{$db{$key}}; $record_num++) {
      foreach $field (@{$db{$key}[$record_num]}) {
         printf("Record %d of %s is %s.\n",
            $record_num,
            $key,
            $field,
         );
      }
   }
}
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foreach $key (keys(%db)) {
   foreach $record (@{$db{$key}}) {
      foreach $field (@$record) {
         ...
      }
   }
}
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To append a record to a given key,

# From ref to array of fields:
push(@{$db{$key}}, $record);
-or-
# From individual fields:
push(@{$db{$key}}, [ $field1, $field2, $field3, $field4 ]);
-or-
# From a my'd array of fields:
{ my @fields = ...; push(@{$db{$key}}, \@fields); }
-or-
# From a array of fields that will change:
push(@{$db{$key}}, [ @fields ]);
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Comma-delimited:

# Gives:
#    key1,field1,field2,field3,field4
#    key1,field1,field2,field3,field4
#    key1,field1,field2,field3,field4
#    key2,field1,field2,field3,field4
#    key3,field1,field2,field3,field4
#    key3,field1,field2,field3,field4
#
# Doesn't handle commas in key or field.

$,=',';
$\=$/;
foreach $key (keys(%db)) {
   foreach $record (@{$db{$key}}) {
      print $key, @$record;
   }
}

-or-

# Gives:
#    key1,0,field1,field2,field3,field4
#    key1,1,field1,field2,field3,field4
#    key1,2,field1,field2,field3,field4
#    key2,0,field1,field2,field3,field4
#    key3,0,field1,field2,field3,field4
#    key3,1,field1,field2,field3,field4
#
# Doesn't handle commas in key or field.

$,=',';
$\=$/;
foreach $key (keys(%db)) {
   for ($record_num=0; $record_num<@{$db{$key}}; $record_num++) {
      print $key, $record_num, @{$db{$key}[$record_num]};
   }
}
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