This is just a golfing of abstract's neat concept. It comes in at 80 chars. Unfortunately, it allows 'ab' to pass, which it shouldn't. I tried to fix it, but couldn't find a way around the s/^((.)\2{$s})// quotemeta thing.

($_,$r,$s)=(pop,-1,0);($r,$s)=($s,$r+$s+1)while s#^((.)\2{$s})##&&!/^\Q$2\E/;!$_