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sub comb {
    map  permute( '', $_, @_ ), ( 0 .. $#_ );
...
       , (0..$#chars))
        : @chars;
}

sub powerset {
    my ($car, @cdr) = @_;
...
#     sort {length $a <=> length $b or $a cmp $b}
      ($car, map("$car$_", @cdr_powerset), @cdr_powerset);
}