Posting a quick update to this as my new and improved understanding of attributes led me to a simple alternative which gave me what I wanted. I realised my main problem was the unless having the wrong effect. The phenomenon of overwriting the fixed length array with a longer one is really just a curiosity, since I don't think there's any practical situation when you'd define an N sized array, and then specify an N + 1 element array as the default. I think the unless issue can be resolved easily in either of 2 ways, depending on what effect is desired:

class Hamper {
    has $.name = 'Christmas Basket';

    my Str @item_defaults[3] = 'Mince Pie', 'White Wine', 'Stinky Chee
+se';

    has Str @.items[3] is rw;

    method TWEAK(){
        @!items[ $_ ] ||= @item_defaults[ $_ ] for @item_defaults.keys
+;
    }
}

my $hamper = Hamper.new;

say "Name: " ~ $hamper.name;
say "Items: " ~ $hamper.items;
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This simply has code in TWEAK assign defaults on a per element basis. This way you get defaults at the element level, so e.g. if you do this:

my $hamper = Hamper.new( items => ('Fish', 'Canoe') );
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$hamper.items will end up with the user specified values Fish and Canoe as the first 2 elements, and Stinky Cheese as the 3rd. This might be good for some purposes, but in my case I wanted the defaults to be completely overwritten if the user specified any values for @!items at all. ie in the above example, asking for $hamper.items should return "uninitialized value" for the third item. Only if I don't try to set any items at all should I get the defaults. This seems quite straightforward to implement with grep:

  method TWEAK(){
      @!items = @item_defaults unless grep {$_}, @!items;
  }
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This now achieves everything I hoped:

• I can insert up to 3 values
• I get an error trying to insert more
• I get an error trying to insert a non-Str value
• I get a set of defaults if I don't specify any at all

I think if you want exactly 3 (or however many) elements, then writing your own accessor is very likely your best option. (Or perhaps subclassing Array? Not really sure of the details there)

Anyway, I just thought I'd share in case it helps anyone else out...