in reply to adding ";" to void elements
$ echo "a;3 b;2;3 c;5;4 d;6" | perl -pe'y/;//==1&&s/$/;/' a;3; b;2;3 c;5;4 d;6; [download]
perl -e 's^$^";"x(2-y-;--)^e' -p data [download]