Found in perldoc perlsub:

  Where people get into trouble is here:

       (@a, @b) = func(@c, @d);
  or
       (%a, %b) = func(%c, %d);

  That syntax simply won't work.  It sets just @a or %a
  and clears the @b or %b.  Plus the function didn't get
  passed into two separate arrays or hashes: it got one
  long list in @_, as always.