While I wholeheartedly agree that you should use a module to parse your CSV, I have to wonder if you wouldn't be better off using Spreadsheet::WriteExcel for your task.

    use Spreadsheet::WriteExcel;

    # Create a new Excel workbook
    my $workbook = Spreadsheet::WriteExcel->new('out.xls');

    # Add a worksheet
    $worksheet = $workbook->add_worksheet();
    $worksheet->write('A24', 'Label');
    $worksheet->write('B24', 'Value1');
    $worksheet->write('C24', 'Value2');
    $worksheet->write('D24', 'Value3');
    $worksheet->write('E24', '=EXACT(B24,D24)');
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