$ perl -MO=Concise -e'1 for $x..$y;'
...
8        <{> enteriter(next->9 last->c redo->9) lKS/8 ->a
...                                               ^
                                                  |
                                       Stacked: Is a counting loop

$ perl -MO=Concise -e'1 for (),$x..$y;'
...
9        <{> enteriter(next->a last->d redo->a) lK/8 ->b
...                                               ^
                                                  |
                                     Not Stacked: Is a foreach loop