This might be gross, but here is the brute force approach that I see:

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

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

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