Before I give some alternative methods, I thought you might like to get the official explanation on the "rows" method - this is 'perldoc DBI' section on "rows":

       "rows"
             $rv = $sth->rows;

           Returns the number of rows affected by the last row affecti
+ng com-
           mand, or -1 if the number of rows is not known or not avail
+able.

           Generally, you can only rely on a row count after a non-"SE
+LECT"
           "execute" (for some specific operations like "UPDATE" and
           "DELETE"), or after fetching all the rows of a "SELECT" sta
+tement.

           For "SELECT" statements, it is generally not possible to kn
+ow how
           many rows will be returned except by fetching them all.  So
+me
           drivers will return the number of rows the application has 
+fetched
           so far, but others may return -1 until all rows have been f
+etched.
           So use of the "rows" method or $DBI::rows with "SELECT" sta
+tements
           is not recommended.

           One alternative method to get a row count for a "SELECT" is
+ to exe-
           cute a "SELECT COUNT(*) FROM ..." SQL statement with the sa
+me "..."
           as your query and then fetch the row count from that.
[download]

As the perldocs suggest, here's an example of doing a SELECT COUNT to get the number of rows found:

  my $sql = {
     SELECT count(*)
     FROM patient_data
     WHERE name = ?
  };
  my $sth = $dbh->prepare($sql);
  $sth->execute($name);
  my ($count_rows) = $sth->fetchrow_array();
[download]

or, you could fetch all the rows, one-by-one, and count them as you go, like:

  my $sql = {
     SELECT name
     FROM patient_data
     WHERE name = ?
  };
  my $sth = $dbh->prepare($sql);
  $sth->execute($name);
  my $count_rows = 0;
  while (my ($name) = $sth->fetchrow_array()) {
     ### do something with $name here ###
     $count_rows += 1;
  }
[download]

Of course, I haven't done anything with error trapping the DBI statements here, but you definitely should.

HTH.