You don't have to do as much as you think by way of doing reading and writing yourself, since Text:CSV_XS's getline returns an arrayref that you can manipulate.

    # parse the row and handle any errors
    my $in_values = $parser->getline($in_fh);
    croak("Unable to parse line $line_num of file $file: "
      . $parser->error_input()) if !$in_values;

    #just add...
    for (@$in_values) { s:\\\\n:\n:gso }
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With the addition of one line, you've unescaped your newlines. And, of course, to put them back, you'd reverse the substitution right before your $csv->print. Ideal? No. But it works, and the performance hit is negligible.