I have updated/inserted CLOBs sucessfully with this syntax:

$sth = $dbh->prepare("update articles set content=? where article_id=?");
# %fdat is a hash set up by Embperl....
$sth->bind_param(1, $fdat{content}, {ora_field=>'content', ora_type=>112});
$sth->bind_param(2, 2442);
$sth->execute();