my $sth = $dbh->prepare("
   SELECT COUNT(DISTINCT all_id) AS cnt
     FROM users
");
$sth->execute();
my $count = $sth->fetchrow_hashref()->{cnt};
$sth->finish();

##</code><code>##

my $sth = $dbh->prepare("
   SELECT COUNT(DISTINCT all_id)
     FROM users
");
$sth->execute();
my $count = $sth->fetchrow_array();
$sth->finish();

##</code><code>##

my $count = $dbh->selectrow_array("
   SELECT COUNT(DISTINCT all_id)
     FROM users
");