my @alpha = ('a'..'c');
my @counter = ();

sub get_next {
  for (@counter) { 
    $_++;
    last if ($_ <= $#alpha);
    $_ = 0;
  }
  push @counter,0 unless grep($_>0,@counter);
  return join"",map($alpha[$_],@counter);
}

for ($_ = get_next();4 > @counter ;$_ = get_next()) {
  print "$_ ";
}
__DATA__
output: a b c aa ba ca ab bb cb ac bc cc aaa baa caa aba bba cba aca bca cca aab bab cab abb bbb cbb acb bcb ccb aac bac cac abc bbc cbc acc bcc ccc