Ideally, you should run your tests with a 32bit and a 64bit perl, with threads and without threads enabled (that is, threads enabled in *perl*, doesn't mean your code actually uses threads), with blead-perl, maint-perl and a selection of older versions of perl (5.8.1, 5.8.0, 5.6.2, ...). With and without taint-checking enabled. With and without PERL_HASH_SEED set. With and without PERL_SIGNALS set to unsafe. All possibilites for -C (or PERL_UNICODE). And in all combinations.