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);
}
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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.