Read perlref. Again. Play with the structures.

Your %HOH corrected:

%HOH = ( # not [ !
   1 => {
#      key => 1, 
      person => "Mike",
      possession => "wallet, keys, car, house",
      age => 25,
   },
   2 => {
#      key => 2, 
      person => "Mike",
      possession => "dog, cat, baseball bat",
      age => 25,
   },
   3 => {
#      key => 3, 
      person => "Dave",
      possession => "pony, house, car, keys",
      age => 21,
   },
); # not ] !
[download]

Likewise for your @AOH. Spot the error. Read perldata and perlref, again. Now to your ostensible problem:

How would one consolidate the hash of hashes to another hash of hashes or array of hashes so that if person and age were same, the possession would be a merge of the possession ... resulting in one entry per person/age combination but with all possessions in the array(list) of values for the possession key?

use Data::Dumper;
use strict; use warnings;

my %HOH = ( # not [ !
   1 => {
#      key => 1, 
      person => "Mike",
      possession => "wallet, keys, car, house",
      age => 25,
   },
   2 => {
#      key => 2, 
      person => "Mike",
      possession => "dog, cat, baseball bat",
      age => 25,
   },
   3 => {
#      key => 3, 
      person => "Dave",
      possession => "pony, house, car, keys",
      age => 21,
   },
); # not ] !

my %NewHOH; # consolidated hash.

{ # limit scope of %seen to this block
  my %seen;   # intermediate hash. Key is "person:age"
  for my $key (keys %HOH) { # iterate over the keys of the hash
    my $val = $HOH{$key}; # get the value - an anonymous hash
    my $composite_key = join ":", $HOH{$key}->{person}, $HOH{$key}->{a
+ge};
    if ($seen{$composite_key}++) { # note: post increment!
        # we make the comma-separated string into an array.
        # then we build a hash using the array elements as keys.
        # this avoids duplicates. We later convert that back.
        my @possessions = split /,\s*/, $HOH{$key}->{possession};
        for my $item (@possessions) {
            $NewHOH{$composite_key}->{possession}->{$item}++;
        }
    } else {
        # first occurence of this "person:age" key
        $NewHOH{$composite_key} = $HOH{$key};
        # same as above. Make a hash of the items of "possession"
        $NewHOH{$composite_key}->{possession} = 
            { map { $_, 1 } split /,\s*/, $HOH{$key}->{possession} }; 
+    
        # record the first key, we'll use that to re-key the hash
        $NewHOH{$composite_key}->{key} = $key;
    }
  }
}

# now get rid of composite keys and make the anon arrays
# of the "possession" value into a string
for my $key ( keys %NewHOH ) {
    my $val = delete $NewHOH{$key};   # returns the anon hash
    my $new_key = delete $val->{key}; # old key which was remembered
    $NewHOH{$new_key} = $val;
    $val->{possession} = join ", ", keys %{$val->{possession}};
}

# debug output
$Data::Dumper::Indent = 1;
my $d =  Data::Dumper->new(
    [\%NewHOH],
    ['NewHOH'],
);
print $d->Dump;
__END__
$NewHOH = {
  '1' => {
    'possession' => 'house, keys, baseball bat, cat, dog, car, wallet'
+,
    'person' => 'Mike',
    'age' => 25
  },
  '3' => {
    'possession' => 'keys, house, pony, car',
    'person' => 'Dave',
    'age' => 21
  }
};
[download]

Note that after __END__, the output shows an anonymous hash! See Data::Dumper, map and split. See perlref, perldata.

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