[perl #130271] [BUG] "Cannot unbox a type object (Any) to a str." with $^var and [⊎]

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This code crashes with "Cannot unbox a type object (Any) to a str".

The problem goes away, if I change $^stuff to $_ or to $^stuff.List or to 
|$^stuff


    my $materials = bag flat 'wood' xx 300, 'glass' xx 100, 'brick' xx 3000;

    my @wanted =
        bag(flat 'wood' xx 200, 'glass' xx 50, 'brick' xx 3000) but "house",
        bag(flat 'wood' xx 100, 'glass' xx 50)                  but "shed",
        bag(     'wood' xx 50)                                  but "dog-house";

    say 'I can build...';
    .put for @wanted.combinations.grep: { $materials ≽ [⊎] $^stuff };