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";
}