LLFourn

use v6;

sub deepgrep(&test,@array) {
    @array.map: {
        when Positional { |deepgrep(&test, $_) }
        default { $_ when test $_ }
    }
}

say deepgrep * %% 2,   [[1], 2, [[3,4], 5],

LLFourn

In this particular case you don't even need a custom accessor. You can create extra constraints on your base type constraint with a where clause:

class Wizzard {
    has Int $.mana is rw where * > 0;
}

But if you did want to create a custom accessor you simply have to define a method with the same name and add the is rw trait.

method mana is rw {