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));