Simple evaluator in Perl 6

evaluate.p6

multi ev(Pair ( :$key where 'number',   :value($n)       )) { $n }
multi ev(Pair ( :$key where 'add',      :$value [$l, $r] )) { ev($l) + ev($r) }
multi ev(Pair ( :$key where 'multiply', :$value [$l, $r] )) { ev($l) * ev($r) }
multi ev(Pair ( :$key where 'variable', :value($name)    )) { %*env{$name} }

my %*env = a => 3, b => 4, c => 5;

my $tree = :add[:variable<a>, :multiply[:number(2), :variable<b>]];

say ev($tree);