Changed the version of minimum perl to 5.008001.

If a support of perl5.6 is dropped, it is better to use a 5.8.1 or newer perl support.

See The Lancaster Consensus
https://github.com/Perl-Toolchain-Gang/toolchain-site/blob/master/lancaster-consensus.md

This will allow test-more modules to reliably use PerlIO and improved Unicode support.