jetzt habe ich doch die richtige stelle in perldata gefunden:
      List assignment in scalar context returns the number of
      elements produced by the expression on the right side of
      the assignment:

          $x = (($foo,$bar) = (3,2,1));       # set $x to 3, not 2
          $x = (($foo,$bar) = f());           # set $x to f()'s return count

      This is handy when you want to do a list assignment in a
      Boolean context, because most list functions return a null
      list when finished, which when assigned produces a 0,
      which is interpreted as FALSE.