You could use the DBI method fetchall_arrayref() and then count elements. There is also a rows() method, but I hear this is not reliable. perldoc DBI
is a good place for more info about these methods.
But depending on *why* you need to know how many rows are returned, the best solution may not be in Perl or DBI but in your database. You should either consider using SQL statements like "select count(*) from poll where ..." and checking that the count is one, or consider building your table keys (using uniqueness) so that the criteria of this sort of SQL statement can return only one row.