fetchrow_array() returns an array. If you assign it like:
my $count = $sth->fetchrow_array();
$count will always equal [the] scalar context of the array
Subroutines return lists not arrays. A list is a series of scalars, an array is a bag that contains a list.
Therefore, $count will be the scalar context of the list. Lists in scalar context evaluate to the their rightmost element. If you actually wanted the behavior you describe above, you'd need to put it inside a "list bag", more commonly called an array:
my $count = @{[ $sth->fetchrow_array() ]};
In this case $count would be the number of elements returned, namely 1.
-Blake