Apparently because -

Use of implicit split to @_ is deprecated
    (D deprecated) It makes a lot of work for the compiler when you
    clobber a subroutine's argument list, so it's better if you assign
    the results of a split() explicitly to an array (or list).
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I think this implies that when you don't specify a target array to split, @_ is used, clobbering the sub args. Must have been deprecated to avoid this happening unintentionally.