chase s/eval/EVAL/ change in Perl 6

Just because I think they change is silly doesn't mean I don't have to change
my code.
chase .bless syntax

Used to be you had to provide a mostly-unnecessary first * argument. We now
handle those bits with reprs instead; much better.
upgrade .exists method to :exists adverb

See also discussion at http://irclog.perlgeek.de/perl6/2014-07-18#i_9046104
for the interesting gotcha I ran into along the way.
change given/when to if/elsif/else

Something changed in 'when' match semantics (or maybe what .parse returns) such
that things matched when they shouldn't. This change fixes the problem, and is
slightly easier to read, too.