deal with perl 5.18's annoying warnings

This deals with the new warnings in 5.18 that cause U-T's tests to fail.

My goal was to remove the use of ~~/given/when and `my $_` from the code in Unicode-Tussle, but to disable the warnings in code provided to `unichars` as shown in the examples.  While I think that it might be nice to replace the examples with `~~`-free ones, I was constrained by time available.