for (@usercodelist) {
  my $SQL = "UPDATE mytable SET subscribed = '0', unsubscribedby = 'SYSTEM' WHERE usercode = '$_' AND unsubscribedby = ''";
  my $sth = $DBH->prepare($SQL);
  $sth->execute || die "Could not execute SQL statement ... maybe invalid?<BR>$!";
  $sth->finish();
}