Meilah, Chapter 5, Mishnah 2. Our chapter is working through the conditions that must be met before a person is held liable for me'ilah. The previous mishnah established the basic principle: when we are dealing with an item of hekdesh whose normal use causes it wear or damage, a person does not transgress me'ilah simply by using it. The item itself must actually be diminished. Our mishnah takes the next step and asks about measurements: how much benefit, and how much damage, are needed before liability attaches?
Half a perutah and half a perutah
The mishnah begins: "Neheneh bechatzi perutah upagam bechatzi perutah" - someone derived half a perutah's worth of benefit from a consecrated item, and also caused it half a perutah's worth of damage.
Picture a garment belonging to hekdesh. A person put it on and wore it. The benefit he received is measured by what it would have cost to rent such a garment, and here that value came to only half a perutah. In the course of wearing it he also tore it slightly, and the loss to the garment likewise amounted to half a perutah. He has benefit, and he has damage, but neither one reaches the threshold of a full perutah.
A perutah here and a perutah there
The mishnah then presents a second case: "O sheneheneh bishaveh perutah bedavar echad upagam bishaveh perutah bedavar acher" - or he derived a full perutah's worth of benefit from one consecrated item, without damaging that item at all, and he caused a full perutah's worth of damage to a different consecrated item, without deriving any benefit from it whatsoever.
Here the full measure is present twice over, but it is split between two separate objects. The benefit sits with one item and the damage sits with another.
For both situations the mishnah issues a single verdict: "harei zeh lo ma'al" - no me'ilah has been committed.
The general rule
The mishnah now states the principle behind both cases: "ad sheyehaneh bishaveh perutah veyifgom bishaveh perutah bedavar echad." Liability for me'ilah does not begin until both conditions are satisfied together: a full perutah's worth of benefit to the person, and a full perutah's worth of damage to the object, and both of them in one and the same item.
Half measures do not combine to make a whole, and benefit taken from one consecrated item does not join with damage caused to another. Only when the single item that was diminished by a perutah is the very same item from which a perutah of benefit was drawn does the transgression take effect.
And it bears repeating that this entire discussion concerns the category of hekdesh items that are ordinarily worn down through use. That is precisely the case in which me'ilah is measured by damage, and therefore the case in which these thresholds apply.