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;
  }
[download]

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...

Hi Tom... with you on the SO tone ... here's what I think is going on:

1. An empty Array is falsey, yet an empty shaped Array is truthy (I would be interested to know why!) - thus, as you have already noted, the original error message is wrong in the suggestion for TWEAK. (You may wish to raise a bug issue at rakudostar on git.) More at https://stackoverflow.com/questions/67373726
2. You are getting a compiler warning from trying to say ~$hamper.items where there is an empty value (Any) in a shaped Array... this does not happen with a non-shaped Array as the length auto-adjusts to the contents.

   Use of uninitialized value element[2] of type Any in string context.
   Methods .^name, .raku, .gist, or .say can be used to stringify it to s
   +omething meaningful.
     in block <unit> at test.raku line 14
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   So, I propose you implement a Str method to bypass (Any) elements and format the object output for .put (or say ~$hamper).
3. Array elements are each scalar containers and are each rw in any case - has @.items is rw just allows the top level array to be assigned to via a generated setter method. (NB. TWEAK can use the private @!items variable directly to assign the defaults).
4. You can see this now gives low boilerplate accessors that do what you want (?) and protect from writing beyond 3 elements.

class Hamper {
    has $.name = 'Christmas Basket';
    has @.items[3];

    method TWEAK {
        @!items = ['Mince Pie', 'White Wine', 'Stinky Cheese'] unless 
+@!items.any.so;
    }   
    method Str {
        "Name:  {$!name}\nItems: [{@!items.grep(*.so).join(', ')}]\n";
    }   
}

given Hamper.new {
    .items[2] = 'Hard Cheese';
    .put;
}
#Name:  Christmas Basket
#Items: [Mince Pie, White Wine, Hard Cheese]

given Hamper.new( items => ['Fish', 'Canoe'] ) { 
    .put;
    .items[2] = 'Bicycle';
    .put;
    #.items[3] = 'Horse';     #fails.. Index 3 for dimension 1 out of 
+range (must be 0..2)
    #.items.push: 'Blue Nun'; #fails.. Cannot push a fixed-dimension a
+rray
}
#Name:  Christmas Basket
#Items: [Fish, Canoe]

#Name:  Christmas Basket
#Items: [Fish, Canoe, Bicycle]
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An empty Array is falsey, yet an empty shaped Array is truthy

I will hazard a guess that Raku handles arrays in scalar context similarly to Perl 5: an array evaluated in scalar context produces the number of elements in the array. An empty array has zero elements, but a shaped array has a fixed number of elements. In boolean context, zero is false and any non-zero number is true.

Thus, an empty array (with zero elements) is a false value, but an array with a fixed non-zero number of elements is a true value.