Quick solution <if anybody still needed on sending json for runmode>

sub send_json_data: Runmode {
  my $self = shift;
  $self->header_add(-type=>"application/json");
  ...
}
[download]

OR refer this inside of CGI::Application::Plugin::JSON import() >> _send_headers() should take care of it.. though, its not taking care of for above runmode to return json data.

sub _send_headers {
    my $self = shift;
    my $private = $self->param('__CAP_JSON') || {};

    if( defined $private->{header} ) {
        $self->header_add( '-x-json' => $self->json_header_string );
    }

    if( defined $private->{json_body} ) {
        $self->header_add('-type' => 'application/json');
    } elsif ( defined $private->{json_callback} ) {
        $self->header_add('-type' => 'text/javascript');
    }
}
[download]