The gemara (Shabbos 10b) quotes Rav as teaching that if you give a gift to a friend, you have to tell him about it -- don't just leave an anonymous present in his mailbox. Maharal explains that by definition a gift is meant to express love, appreciation, etc., to create a bond between giver and recipient. Unless the two parties are aware of each other, that connection is lost. However, writes the Maharal, the mitzvah of tzedaka is simply to fulfill the needs of the poor. Therefore, it can be fulfilled even when the recipient and giver are unaware of each other.
My wife is learning Maharal so I told her to take a look. She immediately threw back at me the pasuk, “Matan b’seiser yichpeh af,” (Mishlei 21) but Tosfos beat her to the punch. Tosfos explains, the gemara is speaking specifically about a present sent to express affection, not just any gift. That’s exactly why a gift of charity can be given secretly.
Why do I bring up this Maharal a week before Purim? Because R’ Yosef Engel uses it as the basis for a tremendous chiddush. The mitzvah on Purim is called “matanos la’evyonim.” It’s a gift! Therefore, unless you know who you are giving to and unless the recipient knows whom he/she is receiving from, you are not yotzei the mitzvah.
Not all poskim are happy with this. Why not compare matanos l’evyonim to the mitzvah of tzedakah, which can be done anonymously? That question gets to the very crux of the debate: is the mitzvah of matanos la’evyonim just a subcategory of the mitzvah of tzedaka that happens to be connected to Purim, or is it a unique mitzvas ha’yom of Purim that operates with its own rules? The fact that we can point to nafka minos between matanos l’evyonim and tzedaka – even the poor are obligated in matanos la’evyonim, the shiur of matanos la’evyonim is different than the shiur for tzedaka, and others – lends weight to the latter position. On the other hand, the purpose of the mitzvah of matanos la'evyonim seems consistent with the purpose of mitzvas tzedaka, and I would venture to guess that most people would equate the two.