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$VAR1 = [
          {
...
            'datetime' => '1310146917'
          }
        ];

[ $id1 => [ { $hashrefs_with_id1 } ... ]
  $id2 => [ { $hashrefs_with_id2 } ... ]
]

$VAR1 = {
          '2446438717782054' => {
...
                                  'datetime' => '1309966331'
                                }
        };

my @row = $r->hashes; # produce flat format of arrays of hashrefs
my %row;
...
  push @{$row{$row->{id}}}, $row;
}
warn Dumper(\%row);