If you are talking about creating a new csv, then just put it there in your print statement. If you are talking about an existing csv, then read the file in and print it back out with a comma appended to the end.
open (IN, $file);
open (OUT, >$temp);
while (<IN>) {
print OUT "$_,\n";
}
close (IN);
close(OUT);
unlink ($file);
rename ($temp, $file);