Using the perpetual system, you keep track of the inventory as you use it. Anyone who took anything from the inventory would need to record it at the moment they took it. Counting supplies at the end of the day or the week or the month still falls under the definition of a periodic system because you are counting it all "periodically." Let's see...perpetual means that you constantly keep track of anything coming or going immediately.
Think about today's technology. Which system is most common?