Be aware that $sth->rows returns sensible results only when used with row-affecting SQL statements.
Check the DBI docs:
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.