$ cat test.pl

use 5.18.2;
use warnings;
use Text::CSV_XS qw( csv );

csv (in => csv (
        in             => *ARGV,
        blank_is_undef => 1,
        on_in          => sub { $_ //= "NULL" for @{$_[1]}; },
        ),
     );

$ cat test.csv
1,2,,3,4
1,2,"",3,4

$ perl test.pl test.csv
1,2,NULL,3,4
1,2,,3,4