And what happens if your data has quotes in it? Here is a version that handles that with the quoting rules used by the CSV format:

sub list2csv {
  my $line = join ",", map {
    my $t = $_;
    $t =~ s/"/""/g;
    $t =~ /\W/ ? qq("$t") : $t;
  } @_;
  $line .= "\n";
  return $line;
}
[download]

Going in reverse is left as an exercise for the reader. Note for the unwary, what about trailing commas, embedded commas, and embedded returns? :-)