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();
}

##</code><code>##

Field           DataType
-----           --------
subscribed      tinyint(1)
usercode        varchar(32) binary
subscribedby    varchar(32) binary
unsubscribedby  varchar(32) binary
unsubscribedby  varchar(32) binary

##</code><code>##

subscribed  usercode                         subscribed by  unsubscribed by
1           7436692b91879380d9108739eb9865f5 SYSTEM