So sayeth perldata:

LISTs do automatic interpolation of sublists....

The null list is represented by (). Interpolating it in a list has no effect. Thus ((),(),()) is equivalent to (). ...

This interpolation combines with the facts that the opening and closing parentheses are optional (except when necessary for precedence) and lists may end with an optional comma to mean that multiple commas within lists are legal syntax. The list 1,,3 is a concatenation of two lists, 1, and 3, the first of which ends with that optional comma. 1,,3 is (1,),(3) is 1,3 (And similarly for 1,,,3 is (1,),(,),3 is 1,3 and so on.) Not that we'd advise you to use this obfuscation.

Combined, of course, with the knowledge that the => is just a comma that quotes its LHS explains it.

Update
Expanded the quote to make it clearer.

It's DWIMery in action. The => fat comma works the same way a comma works with respect to empty args. In other words, @array = ( 1 ,, 2, 3 ); creates a three elemnt list too (not four). If you want an undef element spell it out: undef.