articles/perl.md: add factorial example

articles/perl.md

@@ -59,6 +59,28 @@ print "Hello, World!\n";
 say "Hello, World!";
 ```
 
+### Factorial
+```perl
+use warnings;
+use strict;
+use bignum;     # optional
+
+sub fact {
+	my $n = shift;
+	my $res = 1;
+	while ($n > 0) {
+		$res *= $n;
+		--$n;
+	}
+	return $res;
+}
+
+my @ns = (5, 0, 39, 1, 4, 10);
+foreach my $n (@ns) {
+	printf "fact(%d) = %s\n", $n, fact($n);
+}
+```
+
 ### As a replacement for `sed`
 It is possible to use Perl as `sed` to benefit from its more powerful regex:
 ```sh