my @set = split //, "ABCDEF";
my $len = 12;
my $max_reps = 2;

say gen(\@set, $len, $max_reps) for (1..20);

sub gen {
  my ($set, $len, $max_reps) = @_;
  my $out = '';
  my $last_ix = -1;
  my $reps = 0;
  for (1..$len) {
    my $ix;
    if ($reps >= $max_reps) {
      $ix = int rand(@$set - 1);
      $ix++ if $ix >= $last_ix;
    }
    else {
      $ix = int rand(@$set);
    }
    if ($ix == $last_ix) {
      $reps++;
    }
    else {
      $last_ix = $ix;
      $reps = 1;
    }
    $out .= $set->[$ix];
  }
  $out;
}