use Scalar::MoreUtils qw(zip); # I'm lazy

sub sum_group_by {
    my ($key_columns, $aggregate_columns, $rows) = @_;

    my %result;

    for my $input_row (@$rows) {
        # Create the synthetic key, assuming that no value will contain a \0:
        my @key_values = @{ $input_row }{ @$key_columns };
        my $key = join "\0", @key_values;

        # If this is a new key, create an new result row for it that contains the key values:
        $result{ $key } ||= { zip @$key_columns, @key_values };

        for my $aggregate (@$aggregate_columns) {
            $result{ $key }->{ $aggregate } += $input_row->{ $aggregate };
        };
    };

    # Return the result as new rows, ordered asciibetically (because I'm lazy) by their key:
    map { $result{ $_ } } sort keys %result;
};