while(<INFILE1>) {
    @file_line_contents = split (/\,/,$_);

    # this assumes that $file_line_contents[3] never contains the value '0'
    $file_line_contents[3] ||= $file_line_contents[4];
    # now get rid of the unneeded element (assumes this is the last element in the array)
    pop @file_line_contents;

    print OUTFILE1 "\"$coid\"\,\"".join ('","', @file_line_contents);
}