Start by reading perlintro, and perlsyn. You could use the <> operator (or if you have Perl 5.22, the <<>> operator) to read from the file. You may want to chomp your input. Then split into an array, do your test, warn if there's a problem, and finally print your output.

If you prefer to eliminate shell redirection, then you will also need to open an output file and print to it instead.

If your CSV could ever become non-trivial, you would probably want to switch to using Text::CSV instead of simple split to parse the file.