Note that just as in C, Perl doesn’t define when the variable is
incremented or decremented. You just know it will be done sometime
before or after the value is returned. This also means that modifying a
variable twice in the same statement will lead to undefined behaviour.
Avoid statements like:

    $i = $i ++;
    print ++ $i + $i ++;

Perl will not guarantee what the result of the above statements is.