Try adding a SELECT, to see whether the WHERE clause is matching the records at all...

  my $SELECT = "SELECT * FROM mytable WHERE usercode=? AND unsubscribe
+dby = ''";
  my $UPDATE = "UPDATE mytable SET subscribed = '0', unsubscribedby = 
+'SYSTEM' WHERE usercode=? AND unsubscribedby = ''";
for (@usercodelist) {
  my $sth = $DBH->prepare($SELECT);
  my $q = $sth->execute($_);
  while (my $r = $q->fetchrow_hashref()) {
    print "Matched $$_{usercode}\n";
  }
  $sth = $DBH->prepare($UPDATE);
  $sth->execute($_) 
    or die "<div>Could not execute SQL statement ... maybe invalid?</d
+iv><div>$!</div>";
  $sth->finish();
}
[download]

If you don't get any "Matched" lines, then the WHERE clause is the problem. If all the correct records get "Matched" lines, then the WHERE clause is not the problem.

$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}}
split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/
[download]