I didn't learn Perl, but in general, you can write a recursive
function to calculate factorials, and simply call it in a
statement.
Example (in PHP):
function f($x) {
if ($x > 0) {
return $x * f($x - 1);
} else {
return 1;
}
}
$answer = f(1) + f(2) + f(3) + f(4);