@a / 2 gives you half the number of elements in @a. Which isn't even close to what the OP wants to achieve. You can BTW assign such a result back:

$ perl -wE '@a = "a" .. "z"; @a = @a / 2; say @a'
13
[download]

But @a /= 2 will fail. However, it has little to do what the OP wants.