Syntax you use is not correct. Populating a hash with square brackets is amost 100% wrong:

# WRONG
%HOH = [ ... ];
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If key is always unique, why it's not the hash key? Read the comments to understand the steps I'd take:

#!/usr/bin/perl
use warnings;
use strict;

use Data::Dumper;

my @AoH = (
   {

      key => 1,
      person => "Mike",
      possession => "wallet, keys, car, house",
      age => 25,
   },
   {
      key => 2,
      person => "Mike",
      possession => "dog, cat, baseball bat",
      age => 25,
   },
   {
      key => 3,
      person => "Dave",
      possession => "pony, house, car, keys",
      age => 21,
   },
);

# Make "key" the hash key:

my %HoH;
for my $hash (@AoH) {
    my $key = delete $hash->{key};
    $HoH{$key} = $hash;
}
print Dumper \%HoH;

# In fact, using an array for the possessions would be even better.

for my $hash (@AoH) {
    $hash->{possession} = [ split /, /, $hash->{possession} ];
}
print Dumper \@AoH;


# You don't need any key. You want to hash by person and age. Using pu
+sh merges Mike's possessions.
my %HoH2;
for my $hash (@AoH) {
    push @{ $HoH2{ $hash->{person} }{ $hash->{age} } }, @{ $hash->{pos
+session} };
}
print Dumper \%HoH2;
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