Pretend the text is in Chinese (or something else you can't understand), all the writing is on notepads and that you're so drunk you can't remember what you did 10 seconds ago.
Now, write down a set of instructions so that your imaginary self could accomplish the task of copying the desired symbols from the first notepad to the stack of blank ones.

Replace "notepad" with "file" and the algorithm should now be easy to translate into perl.