The arguments to a function are being interpreted as a list, and successive commas do not act as a placeholder for empty elements (yes, I've also at times wished this would work...).

From perldata:

The null list is represented by (). Interpolating it in a list has no effect. Thus ((),(),()) is equivalent to (). Similarly, interpolating an array with no elements is the same as if no array had been interpolated at that point.

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.)

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