Suppress warnings on Perl-5.18

I got many warnings like "Use of each() on hash after insertion without
resetting hash iterator results in undefined behavior".

This problem is same as following URL.
 - https://github.com/tokuhirom/Module-Functions/pull/2
Merge pull request #4 from syohex/fix-for-5.18

Suppress warnings on Perl-5.18
(Thanks a lot @syohex)