on many RDBMS the execute method will return the number of rows like so:

my $query = "SELECT * FROM myTable";
$sth = $dbh->prepare($query);

if(!$sth->execute())
{
...
}
[download]

it varies from system to system .. but is another way to accomplish what you want

edit: you could also run this query
SELECT Count(yourPrimaryKeyHere) FROM yourTable
if you're on a database that doesn't support execute's return value or $sth->rows