my $L = 4;
my @chars = qw[a b c d];
$_ = "aaaabdbbdaccdacdaaabdcd";

my %seen = map {$_, 0} glob do {local $" = ","; "{@chars}" x $L};
{local $" = ""; use re 'eval'; /([@chars]{$L})(?{$seen{$1}++})(*FAIL)/;}

while (my ($n, $v) = each %seen) {
    say "$n -> $v";
}