Parens are usually used because "=" has higher precedence than ",".

>perl -MO=Deparse,-p -e "@a = 1, 2"
((@a = 1), '???');
-e syntax OK

>perl -MO=Deparse,-p -e "@a = (1, 2)"
(@a = (1, 2));
-e syntax OK
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('???' is the 2 in void context being optimized away.)