$sth = $dbh->prepare(qq{
    INSERT INTO people (fullname, age) VALUES (?, ?)
  });
  $sth->execute("Joe Bloggs", undef);

##</code><code>##

WHERE ISNULL(age, xx) = ISNULL(?, xx)

##</code><code>##

$where_clause = defined $age? "age = ?" : "age IS NULL";