I've gotta be missing something or smoking too much crack.

Well, I don't really find that beaming up effects my coding adversely, so my guess is that you're missing something.

Just a hunch, but I bet you have newlines attached to the values in @usercodelist. FWIW, you ought to prepare that statement once with a placeholder, and repeatedly execute it.

Try this:

my $SQL = "UPDATE mytable SET subscribed = '0',
                          unsubscribedby = 'SYSTEM'
           WHERE usercode = ? AND unsubscribedby = ''";

my $sth = $DBH->prepare( $SQL );

foreach my $md5 ( @usercodelist ) {
    chomp $md5; # I'm assuming a newlines is bungling your update
    $sth->execute( $md5 ) or die "Can't execute $SQL";
}
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-- am