get '/test_download' => sub {
    return send_file (
        'data.csv',
        streaming => 1,
        callbacks => {
            override => sub {
                my ( $respond, $response ) = @_;
                my $writer = $respond->(200, [ 'Content-Disposition' => 'attachment; filename="data.csv"' ] );
                for ( 0 .. 100) {
                    my $line = join ',', $_, qw(1 2 3 foo bar baz 47 3.14159);
                    $writer->write ($line . "\n");
                }
            },
        },
    );
};