use v5.18; # http://math-prosto.ru/?page=pages/factorial/factorial.php sub fac { my ($n) = @_; return 1 if $n == 0 || $n == 1; my $res = 1; for (my $i = 1; $i <= $n; $i++) { $res *= $i; } return $res; } say fac 0; say fac 1; say fac 5;