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

sub permute {
    my ( $str, $depth, @chars ) = @_;
    map $str.$_,
      $depth
	? (map permute( $chars[$_],
			$depth-1, @chars[ ( $_ + 1 ) .. ($#chars) ] )
	   , (0..$#chars))
        : @chars;
}