This is actually pretty well-documented in perldata:

This interpolation combines with the facts that the opening and closing parentheses are optional (except 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.

If you take that logical explanation into account I think you don't want to make perl complain over two commas directly following each-other.

(However, I wonder if that documentation really is true. To me it seems like 1,,,3 "really" is ((1,),),3 is (1,),3 is 1,3. Futher, afaik the comma operator needs something to the left. In (,) it's got nothing.)

-Anomo

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