From the documentation for split:

In scalar context, returns the number of fields found and splits into the @_ array. Use of split in scalar context is deprecated, however, because it clobbers your subroutine arguments.

And the output I get from use diagnostics:

Use of implicit split to @_ is deprecated at test.pl line 7 (#1).
(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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