Streaming in any form was the main problem that I had w/Dancer 1. While moving to Dancer 2 was partially influenced by that, it was also because Dancer 2 has nicer routing and other features.

The current code looks like:

sub proxy_request {
    my $self;
    $self = shift if ( ( _whoami() )[1] ne (caller)[1] );
    my ( $proxy_route, $file_name ) = @_;

    unless ($proxy_route) {
        Tranquillus::Util->return_error('BAD_QUERY');
    }

    my $cb = sub {
        my $respond = $Dancer2::Core::Route::RESPONDER;
        require LWP;
        my $ua = LWP::UserAgent->new;
        my $writer;
        my %m;

        $ua->get(
            $proxy_route,
            ':content_cb' => sub {
                my ( $data, $response, $protocol ) = @_;

                if ( not $writer ) {
                    my $h = $response->headers;

                    my @ary =
                        ( 'Cache-Control', 'Content-Length', 'Content-
+Type', 'Last-Modified', 'Content-Disposition' );

                    foreach my $key (@ary) {
                        if ( $h->header($key) ) {
                            $m{$key} = $h->header($key);
                        }
                    }
                    unless ( exists $m{'Content-Length'} ) {
                        # RFC 7230
                        # http://tools.ietf.org/html/rfc7230#section-4

                        $m{'Transfer-Encoding'} = 'chunked';
                    }

                    if ($file_name) {
                        $m{'Content-Disposition'} ||= 'attachment; fil
+ename="' . $file_name . '"';
                    }
                    $writer = $respond->( [ $response->code, [%m] ] );
                }
                # Ensure that we have a valid writer...
                if ($writer) {
                    $writer->write($data);
                }
            },
        );
        # Cleanup.
        # Ensure that we have a valid writer...
        if ($writer) {
            if ( exists $m{'Transfer-Encoding'} ) {
                $writer->write(undef);
                $writer->write("\r\n");
            }
            $writer->close;
        }
    };

    my $response = Dancer2::Core::Response::Delayed->new(

        #            error_cb => sub { $weak_self->logger_engine->log(
+ warning => @_ ) },
        cb       => $cb,
        request  => $Dancer2::Core::Route::REQUEST,
        response => $Dancer2::Core::Route::RESPONSE,
    );

    return $response;
}
[download]

... and can be found at: https://github.com/gsiems/tranquillus/blob/master/lib/Tranquillus/Proxy.pm

Update: fixed the github link.