sub printsubsets2 {
  my $stuff = join('',@_);
  my $mask = "\xff" . ("\x00" x (length($stuff)-1));
  do {
    my $str = $mask & $stuff;
    $str =~ y/\x00/ /;
    print $str,"\n";
    if ($mask =~ s/\xff\x00(\x00*)$/\xff\xff$1/) {}
    elsif ($mask =~ s/\xff\x00(\x00*)\xff$/\x00\xff$1\x00/) {}
    elsif ($mask =~ s/\xff{2}$/\x00\xff/) {}
    else {return;}
  } while (1);
}

printsubsets2(qw(a b c d e));