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Should one have a fixed, formal meal for Melava Malka on Motzei Shabbos?
An idea (lechatchila), yes, and meat as well. Though they are not strict requirements. (Mishna Berura 300:1, Aruch haShulchan 300:3)
At the very least, one should eat a kezayis of bread. If not possible, a kezayis of mezonos or fruit will also do, but to eat bread is much better.