In SVN 3920 was committed a patch that does this:

# Pass this back to the caller.
$p0 = $playlistObj;

The CLI will echo $p0 back to the caller and in this case is will not 
be "playlist" as it should but

playlist save tt
file%3A%2F%2F%2FUsers%2Ffred%2FMusic%2FPlaylists%2 Ftt.m3u save tt

Given the architecture of Command:execute (that we can discuss), the 
way to achieve what you are looking for is an explicit callback, I 
believe. You can't assign anything to $pX that you do not want printed.