my $all = $dbh->selectall_arrayref("
    SELECT ID, col1, col2
      FROM table
     WHERE col1 = 'xxx'
");

print "Number of rows selected: ", 0+@$all, "\n";

##</code><code>##

my ($count) = $dbh->selectrow_array("
    SELECT COUNT(*)
      FROM table
     WHERE col1 = 'xxx'
");

print "Number of rows selected: $count\n";