1 use List::AllUtils qw/sum/;
  2 use Bit::Vector;
  3 
  4 my $k = 2;
  5 
  6 my @letters = split '',"ABCDE";
  7 my $n = scalar(@letters);
  8 
  9 my $vec = Bit::Vector->new($n);
 10 
 11 while(!$vec->increment){
 12     my $i = $vec->to_Bin; # current bitstring
 13     my $s = sum(split '',$i); # sum up to find out how many bits are lit
 14 
 15     next unless ($s == $k); # just the ones with $k bits lit
 16 
 17 
 18     my $str = # convert bitstring to string
 19         join '',
 20         map { $letters[$_]  }
 21         grep { $vec->contains($_);  }
 22         0..$n-1 ;
 23 
 24     print "$s $i $str\n";
 25 };