Fixed a code issue and a test issue

Change 1:
From http://search.cpan.org/~dapm/perl-5.10.1/pod/perlguts.pod#AVs,_HVs_and_undefined_values, using  &PL_sv_undef will cause issue when modify the hash item, or iterate the hash.

Change 2:
The original test for mod 20 will always success as we changed the read buffer from 20 to 1024