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Account    Entity    Unit    Jan    Feb    Mar
Account01    Entity01    Unit01    1    2    3
Account01    Entity01    Unit01    4    5    6
Account01    Entity01    Unit01    7    8    9
Account02    Entity02    Unit02    10    11    12

Account    Entity    Unit    Jan    Feb    Mar
Account01    Entity01    Unit01    12    15    18
Account02    Entity02    Unit02    10    11    12

CREATE TABLE concat_FRA3 AS
  SELECT Account, Entity, Unit, 
    SUM(Jan) Jan,
...
    SUM(Mar) Mar
  FROM concat_FRA2
  GROUP BY Account, Entity, Unit;

# Store
my %storage;
foreach my $row (@$result_set) {
...
    }
    $storage{$hash_key} = $new_row;
}