The multiplication happens after the recursive call in your snippet, so your snippet isn't tail recursive. The following is a tail recursive version of your snippet:
sub _tail_recur {
my ($x, $y) = @_;
if ($x <= 1) {
return $y;
} else {
return _tail_recur($x - 1, $x * $y);
}
}
sub tail_recur {
my ($x) = @_;
return _tail_recur($x, 1);
}
Tail recursion allows for the compiler to automatically flatten recursion (slow, and memory usage is proportional to the recursion depth) into a loop (fast, and memory usage is constant). Unfortunately, Perl5 does not do tail-recursion optimization.