"Useless use in void context" is not a comment about the operands. It means the result of an operation is being discarded without being used. In this case, the result of the concatenation of the value returned by print and what follows is being discarded.

The following would also issue the warning

@a = 4, 5;
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because Perl interprets that as

( @a = 4 ), 5;
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The result of the list operator is being discarded. (I'm making the likely assumption that the above is being evaluated in void context.)