Perhaps this is -too- idiomatic, but for recipe 04-08 (You want to find elements in one array but not in another):
@aonly = gather { take when none(@b) for @a; };
Though if when is considered a statement modifier in this case, and if you can't combine statement modifiers in p6 (much as you cannot in p5), then that likely doesn't work. In that case:
@aonly = gather { for @a { take when none(@b) }};
For numeric @a, and $_ will these be equivalent:
( $_ == none( @a ) ) && say "foo";
( $_ ~~ none( @a ) ) && say "foo";