$insert_sth = $dbh->prepare(<<"EndOfSQL");
  INSERT INTO members
    (member_id, name, date_joined)
  VALUES
    (?, ?, NVL(?, SysDate))
EndOfSQL

$insert_sth->execute(1, 'Bob', '2001-08-15');  # Bob actually joined yesterday
$insert_sth->execute(2, 'Tom', undef);         # Tom is joining now