Tried the below off of the Dancer2 docs, but it returns 'foo':

    return send_file( \'foo',
        streaming    => 1,
        content_type => $mime_type,
        callbacks    => {
            override => sub {
                my ($respond, $response) = @_;
        my $writer = $respond->([ 200, {} ]);

                my $buf = '';
                while( read( $in_fh, $buf, VID_READ_LENGTH ) ) {
                    $writer->write( $buf );
                }
            }
        },
    );
[download]

Running in the debugger shows that the callback never gets called.

... Running in the debugger shows that the callback never gets called.

Well :) time to UTSL good buddy