First, the fetchall_* methods fetch all of the rows

Not always true. The fetchall_arrayref takes a $max_rows optional argument which limits the number of rows fetched in one go to $max_rows. The example shown was in fact using $max_rows. e.g., from the DBI docs:

If $max_rows is defined and greater than or equal to zero then it is used to limit the number of rows fetched before returning. fetchall_arrayref() can then be called again to fetch more rows. This is especially useful when you need the better performance of fetchall_arrayref() but don't have enough memory to fetch and return all the rows in one go.

Here's an example (assumes RaiseError is enabled):

my $rows = []; # cache for batches of rows
while( my $row = ( shift(@$rows) || # get row from cache, or reload ca
+che:
                            shift(@{$rows=$sth->fetchall_arrayref(unde
+f,10_000)||[]}) )
         ) {
           ...
         }
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