sub combinations returns Array  (@list is rw) {
  return () unless @list.elems;
  my @ans;
  for 1 .. 2**@list.elems-1-> $num {
      push @ans, [ @list[ (0 .. sqrt($num)).grep:{ $num +& (2**$_) } ] ];
  }
  return @ans;
} 
my @list = (1..4);
combinations(@list).perl.say;
__OUTPUT__
([1], [2], [1, 2], [3], [1, 3], [2, 3], [1, 2, 3], [4], [1, 4], [2, 4], [1, 2, 4], [3, 4], [1, 3, 4], [2, 3, 4], [1, 2, 3, 4])