Fat comma would have interpreted lc as a string. Normal comma would have been less pretty.

The whole dereferencing is a bit complex, but it basically uses the @hash{key1, key2} slice syntax. I.e., the first map maps lower case strings from @a to their originals, this is inserted into an anonymous hash, and only the values corresponding to lowercased elements of @b are returned to the outer grep that selects the defined ones.