This could help you improve "MetricPrefix":

my %to_num = map { $_[0]=> 10**$_[1] },
         (qw|pico nano micro milli centi hecto kilo mega giga tera| 
          Z
         (-12, {$_+3}... -3,-2,2,3,{$_+3}... 12)) ;
[download]

Uses "Lazy lists" and the Zip (Z) operator.

I'm still learning p6, and I agree - there HAS to be a better way to avoid all that boilerplate code, so I'll keep a watch for responses.