use strict;
use warnings;

my $letters = 'ABCDEF??';

my $length = length($letters);

## We'll be using factorials a lot, so it's best to pre-calculate
my @factorial = (0,1);
$factorial[$_] = $factorial[$_-1] * $_ for 2..$length;

my ($blanks, @lsets, $lset, $nlset, $words, $twords);

## Find the number of blanks
while ($letters =~ /[^A-Z]/) {
    $letters =~ s/[^A-Z]//;
    $blanks++;
}

## Produce unique letter sets, given blanks
@lsets = ($letters);
while ($blanks--) {
    my %lsets;
    for $lset (@lsets) {
        for ('A'..'Z') {
            $nlset = join '', sort split //, $lset.$_;
            $lsets{$nlset} = ();
        }
    }
    @lsets = keys %lsets;
}

## Calculate the number of words possible from each letter set
for (@lsets) {
    $words = $factorial[$length];
    my %lcount;
    $lcount{$_}++ for split //, $_;
    for (keys %lcount) {
        $words /= $factorial[$lcount{$_}] if $lcount{$_} > 1;
    }
    $twords += $words;
}

print $twords;