Rebecca, as you said, perishable food items may likely be a problem for a food pantry or even a shelter.
But, there are likely some places that will take it, if the timing is right. I know that the synagogue we used to belong to would take the components of a home-cooked meal -- entrees, sides, desserts, bread -- over to a shelter in Somerville one Sunday afternoon a month (sorry, don't remember the name of the shelter). I think a big part of it is that the folks at the shelter/soup kitchen have to be prepared to handle whatever you're bringing.
I'd recommend posing this question to the Arlington list serv, which, as I'm sure you know already, is a fantastic resource.
You might also try calling Town Hall. I know there's an organization called Minuteman Senior Services that the town is involved in, but I don't know if they serve meals or would accept donations.
There's also the Bread and Jams soup kitchen, which is located on Mass. Ave in Cambridge, not far from Porter Square, 617-441-3831.
Here's a link with some other places that might be of help: http://www.bostonhomeless.org/ShelterChoose.asp
I'd also recommend sending Ellyn an e-mail or PM, in case she doesn't see this thread -- she's worked in the social service area for many years, and might have some better leads for you.
(Or, as Kari said -- if you're looking to feed a hungry BB'er, you know where you can find me!)
"We reject as false the choice between our safety and our ideals. Our founding fathers, faced with perils we can scarcely imagine, drafted a charter to assure the rule of law and the rights of man, a charter expanded by the blood of generations. Those ideals still light the world, and we will not give them up for expedience's sake."
--President Barack Obama, 1/20/09