try this:

$sth = $dbh->prepare("select count(distinct all_id) as a from users") 
+or die ("Error in SQL\n");
$sth->execute();
my $counts = $sth->fetchrow_hashref();
print "There are ".$counts->{'a'}." in this table";
[download]

fetchrow_hashref() fetches the next row of data and returns it as a reference to a hash containing field name and field value pairs.