Looking at the source of HTTP::Message, it just does not want to decode the content delivered by a callback:

...
    $content_ref = $self->content_ref;
    die "Can't decode ref content" if ref($content_ref) ne "SCALAR";
...
[download]

Either you fix HTTP::Message or you try it with ->content() instead of ->decoded_content. Alternatively, look at Plack as your server framework - it should handle callback responses more gracefully than your approach.