Dancer2 asynchronous file serving without `send_file` with extra stuff

async-file-serving-without-send_file.pl

use Dancer2;
use Text::CSV_XS  qw< csv >;
use Path::Tiny    qw< path >;
use JSON::MaybeXS qw< encode_json >;

# Create a CSV parser
my $csv = Text::CSV_XS->new({
    'binary'    => 1,
    'auto_diag' => 1,
});

get '/' => sub {
    # Return async response

    delayed {
        # Start async
        flush;

        # Read each row and stream it in JSON
        my $fh = path('filename.csv')->openr_utf8;
        while ( my $row = $csv->getline($fh) ) {
            content encode_json $row;
        }

        # Close connection
        done;
    };
};

If we didn't care about serving it line by line and encoding this in JSON, the following could have been used instead:

get '/' => sub {
    send_file 'filename.csv';
};

send_file automatically uses the asynchronous file serving if it's available.