much easier than that

  
$res->body($io);
Or

use Path::Tiny qw/ path /;
    return [
        200,
        [
            'Content-Type' => 'text/plain; charset=UTF-8',
        ],
        path( __FILE__ )->openr_raw,
    ];
[download]

https://metacpan.org/pod/Plack::Response#body