You should be able to detect the position/state of the video by polling:

my( $value, $type ) = $mech->eval_in_page(<<'JS');
    var videoElement = find_video_element(); // you need to write that
+ yourself
    return videoElement.ended;
JS
[download]

Also of interest could be videoElement.currentTime; to maybe better synchronize your playback.

You could also use more fancy Javascript to send events to a Perl webserver when the playback starts/ends or every five seconds etc.