Remove experimental use of lexical $_ causing failures in more recent Perls

Goodie::Dice was using a lexical $_.  This was an experimental feature added in
Perl v5.10.0 that is now considered to have been a bad idea.  See
http://perldoc.perl.org/perlvar.html#%24_ for more info.

This was causing test failures for me and `duckpan server` would not start.  I'm using Perl v5.19 btw.  I realize this is probably not an issue for DDG as you probably aren't using such a modern version of Perl.  But it could be a problem for other contributors.