If you already feel comfortable with SQL, you can also use an SQLite virtual table to treat your Perl data:

our $aref = $result_set;

$dbh->do(<<'');
CREATE VIRTUAL TABLE my_table USING perl(Account, Entity ,Unit,
                                     arrayrefs="main::aref")

my $elements = $dbh->selectall_arrayref(<<'SQL', undef, {});
  SELECT Account, Entity, Unit, 
    SUM(Jan) Jan,
    SUM(Feb) Feb,
    SUM(Mar) Mar
  FROM my_table
  GROUP BY Account, Entity, Unit;
SQL

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