(a1 + b1) * (a2 * a3 * a4 + a2 * a3 * b4 + ...)

##</code><code>##

(a1 + b1) * (a2 + b2) * (a3 + b3) * (a4 + b4)

##</code><code>##

$sum = 1;
for my $i (0 .. 3) {
    $sum *= $A[$i] + $B[$i];
}
say $sum;

##</code><code>##

my $sum = 0;
for my $pattern (map unpack('b4', pack 's', $_), 0 .. 15) {
    my $prod = 1;
    for my $i (0 .. 3) {
        $prod *= (\@A, \@B)[substr $pattern, $i, 1][$i];
    }
    $sum += $prod;
}
say $sum;