sub printsubsets1 {
  my ($prefix, @elemarray) = @_;
  if (!@elemarray) {return;}
  my $elem = shift @elemarray;
  print "$prefix$elem\n";
  printsubsets1 ("$prefix$elem", @elemarray);
  printsubsets1 ("$prefix ", @elemarray);
}

sub printsubsets { printsubsets1("",@_);}

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


##</code><code>##

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));



##</code><code>##

sub initialmask {
  my $nelem = shift;
  return "\xff" . ("\x00" x ($nelem-1));
}

sub nextmask {
  if ($_[0] =~ s/\xff\x00(\x00*)$/\xff\xff$1/) {1;}
  elsif ($_[0] =~ s/\xff\x00(\x00*)\xff$/\x00\xff$1\x00/) {1;}
  elsif ($_[0] =~ s/\xff{2}$/\x00\xff/) {1;}
  else {0;}
}

# later in your code:

my $mask = initialmask($nfactors);
do {
  # blah blah blah $mask
} while (nextmask($mask));


##</code><code>##

--
@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/;
map{y/X_/\n /;print}map{pop@$_}@/for@/