I didn't explain my statements above very well, it was kind of stream of consciousness. The test code I made was to rule out something in your application. The simple example has the same issues as your program. The error message seems to be coming from SDL::Video::set_video_mode. SDLx::App::resize has the call shown below that should print an error. Since we never see this error the problem is somewhere in this function.

        $self = SDL::Video::set_video_mode( $w, $h, $bpp, $flags )
            or die "SDL cannot set video:" . SDL::get_error;
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The functions in SDL::Video are in a dll so this is as far as I got.