Sweet!

Why do you flatten the array? I played with that in Pugs and the asterisk seems superfluous. It seems like one is copy the elements and the other is copying the array. I thought that maybe your version would avoid aliasing problems, but even in checking that, I'm not seeing it happening.

sub group (@array is copy) {   # didn't flatten
    gather {
        while @array {
            take [
                gather {
                    my $h = shift @array;
                    take $h;
                    while @array and $h eq @array[0] {
                        take shift @array;
                    }
                }
            ];
        }
    }
}
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