$ cat test.pl
use 5.20.0;
use warnings;

use Text::CSV_XS "csv";

csv (in => "test.csv", blank_is_undef => 1, on_in => sub {
    splice @{$_[1]}, 2, 2, $_[1][2] // $_[1][3]; });
$ cat test.csv
1,2,,,5,6
1,2,3,,5,6
1,2,,4,5,6
1,2,3,4,5,6
$ perl test.pl
1,2,,5,6
1,2,3,5,6
1,2,4,5,6
1,2,3,5,6