while ( my $row = $csv_in->getline($fh_in) ) {

    # $row->[2]=100;   # just fooling around
    $fh_out->print(
        "Olah:",
        $number++, "\t", $row->[2], "\t", $row->[3], "\t", " 2 x 3 =  ",
        $row->[2] * $row->[3], "\n"

          # @$row[2,5,3,0,4], # all unchanged columns unchanged
    );
}