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