From type/Routine

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multi sub trait_mod:<is>(Routine:D $r, :$raw!)

Gives total access to the data structure returned by the routine.

my @zipi = <zape zapatilla>;
sub þor() is raw {
    return @zipi
};
þor()[1] = 'pantuflo';
say @zipi;  # OUTPUT: «[zape pantuflo]␤»