Your performance is determined by the type of operations you're performing. If you are not scanning keys (ie, my @keys = keys(%bigHash); such that you are always looking up values with a previously known key (ie, my $value = $bigHash{$key};), then a hash maintains its performance quite well even when it is very large and, as stated early, memory/resource usage is your greatest concern.

That said, Benchmark can help you determine the performance of the hash at different sizes.