That's quite simple. If the only thing you could ever write between the curly and the square bracket is a -> and you have to write it always, then why should you have to write it at all?

It's a convenient shortcut that may be used no matter whether you are chaining array or hash subscriptions. $AoA[1][4], $HoH{some}{thing}, $AoH[1]{whatever}, $HoA{barf}[42], ...

The arrow at the beginning though is important. $A[0] is the first element of the array @A, $A->[0] on the other hand is the first element of the array referenced by $A. Because arrays and hashes may only store scalars (strings, numbers or references) you do not need the arrow between the subscriptions to distinguish such two cases.