Does that mean that the cases are run serially (as in the original Benchmark approach) -- that is, case #2 doesn't start until case #1 is finished?

Correct, the cases are run serially, with the parent process waiting for the forked child to complete the timing cycle before it proceeds.

I don't have any experience using a two-arg open where the second arg is "-|" -- perdoc -f open doesn't discuss this usage directly

You may need to just look harder...

  If you open a pipe on the command '-', i.e., either '|-' or '-|'
  with 2-arguments (or 1-argument) form of open(), then there is
  an implicit fork done, and the return value of open is the pid
  of the child within the parent process, and 0 within the child
  process. (Use defined($pid) to determine whether the open was
  successful.) The filehandle behaves normally for the parent, but
  i/o to that filehandle is piped from/to the STDOUT/STDIN of the
  child process. In the child process the filehandle isn't opened--
  i/o happens from/to the new STDOUT or STDIN.
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I'll update the comments to mention this, and fix the documentation nit you pointed out. Thanks for the feedback!