$dbh->do("update table set thing = ? where id = ?",
            \%attr, $thing, $id);

##</code><code>##

  $dbh->do("update table set thing = ? where id = ?",
            undef, $thing, $id);
  # OR
  $dbh->do("update table set thing = ? where id = ?",
            {}, $thing, $id);