По рекомендации @Phillipos:
sed 'h;s/.*,//;s/./,"&"/g;x;G;s/,.*\n/"/;s/^/"/' inp.csv
Пояснение:
Разделите пространство паттернов на две части и работайте с ними по отдельности, а затем соедините их вместе.
perl -lpe '$_ = q["]. join(q[","], unpack sprintf "A%dx(A)*", index $_, ","). q["]' inp.csv
Краткое пояснение:
° Construct the unpacking format for the input line by finding the location position of the comma, skip the comma from unpack, remaining are unpacked a char each. Then joined with the string ",", finally all enclosed in double quotes.