I think I was not explained my problem clearly. I also do not worry about the return value but the DELETE query behaves differently in execute & do.
$dbHandle->do is not deleting the DB rows. This is what my expected behaviour because , Ex: SESSION_MAX_LIFE_TIME = 900 secs (15mins), and $now is perl's time() function value. I would like to delete session id which is more than 15 mins old. In this case, session ids are just created, it needs to live for 15 mins.
$stHandle->execute just deletes all the sessionsids (all db rows in database) , It should behave like $dbHandle->do right?.