$ cat test.pl
use 5.20.0;
use warnings;

use Text::CSV_XS "csv";

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