sub factorial {
    my ($n) = @_;
    if ($n == 0) { 
         return 1;
    }
    return $n * factorial($n - 1);
 }

##</code><code>##

sub factorial {
    my ($n, $accumulator) = @_;
    $accumulator ||= 1;
    if ($n == 0) { 
         return $accumulator;
    }
    return factorial($n - 1, $n * $accumulator);
}

##</code><code>##

call factorial ...
    call factorial ...
        call factorial ...
            call factorial ...
            return
        return
    return
return

##</code><code>##

call factorial ...
change args and reloop ...
change args and reloop ...
change args and reloop ...
return

##</code><code>##

sub factorial {
    my ($n, $accumulator) = @_;
    $accumulator ||= 1;
LOOP:
    if ($n == 0) { 
         return $accumulator;
    }
    $accumulator *= $n;
    $n--;
    goto LOOP;
}