@items = ( { item_key         => '1',
             item_description => 'description 1',
             item_summary     => 'item summary 1',
             actions          => [ { key => '1',
                                     summary => 'summary 1'
                                   },
                                   { key => '2',
                                     summary => 'summary 2'
                                   }
                                 ]
           },
           ...
         );

##</code><code>##

my $items_sth   = $dbh->prepare('SELECT * FROM Items');
my $actions_sth = $dbh->prepare('SELECT * FROM Actions WHERE item_key = ?');

my @items;
$items_sth->execute($item_row->{item_key});
while (my $item_row = $items_sth->fetch_hashref()) {
   my $item = {
      item_key         => $item_row->{item_key},
      item_description => $item_row->{item_description},
      item_summary     => $item_row->{item_summary},
      actions          => [],
   };

   my $actions = $item->{actions};
   $actions_sth->execute($item_row->{item_key});
   while (my $action_row = $actions_sth->fetch_hashref()) {
      push @$actions, {
         key     => $action_row->{action_key},
         summary => $action_row->{action_summary},
      };
   }

   push @items, $item;
}

##</code><code>##

my @items;
my $last_key;
while ($row = $sth->fetch_hashref()) {
   if (!@items || $last_key != $row->{item_key}) {
      push @items, {
         item_key         => $row->{item_key},
         item_description => $row->{item_description},
         item_summary     => $row->{item_summary},
         actions          => [],
      };

      $last_key = $row->{item_key};
   }

   push @{ $results[-1]{action} }, {
      key     => $row->{action_key},
      summary => $row->{action_summary},
   };
}

##</code><code>##

my %items;
my @items;
while ($row = $sth->fetch_hashref()) {
   our $item; local *item = \( $items{ $row->{item_key} } );
   if (!$item) {
      $item = {
         item_key         => $row->{item_key},
         item_description => $row->{item_description},
         item_summary     => $row->{item_summary},
         actions          => [],
      };
      push @items, $item;
   }

   push @{ $item->{action} }, {
      key     => $row->{action_key},
      summary => $row->{action_summary},
   };
}