You are not very specific with respect to when you are trying to get
the count of rows. However, from the doc, for SELECT queries the
returned value is only valid after all rows have been retrieved.:
$rv = $sth->rows;
Returns the number of rows affected by the last row affecting command,
or -1 if the number of rows is not known or not available.
Generally, you can only rely on a row count after a non-SELECT execute
(for some specific operations like UPDATE and DELETE), or after
fetching all the rows of a SELECT statement.
For SELECT statements, it is generally not possible to know how many
rows will be returned except by fetching them all. Some drivers will
return the number of rows the application has fetched so far, but
others may return -1 until all rows have been fetched. So use of the
rows method or $DBI::rows with SELECT statements is not recommended.
One alternative method to get a row count for a SELECT is to execute a
``SELECT COUNT(*) FROM ...'' SQL statement with the same ``...'' as
your query and then fetch the row count from that.