To me, it is not clear what you want at all. Do you want the result of the split excluding the first element in @_? Or do you want @_ only hold the first element and discard the rest?

$ perl -E'$a="a,b,c,d";(undef,@_)=split/,/,$a;say "@_"'
b c d

$ perl -E'$a="a,b,c,d";@_=(split/,/,$a,2)[0];say "@_"'
a
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