`rows'
  $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.