So what will the exec do?!

I have no idea. :-p

Well, system is basically the same thing as fork/exec and my point was basically to do the STDERR redirect between the fork and the exec so that it wouldn't be (temporarily) redirected in the parent. I suppose that it's not really that big of a deal so you might as well just do:

#
# Note:  UNTESTED!!!
#
sub do_run
{
  # write file...

  local *OLDERR, *NEWERR;
  open(OLDERR, ">& STDERR") or die "Can't dup STDERR: $!";
  pipe(NEWERR, STDERR) or die "Can't pipe: $!";

  system "perl quick_run.perl";

  open(STDERR, ">& OLDERR") or die "Can't restore STDERR: $!";

  my @error_lines = <NEWERR>;
  return @error_lines;
}
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bbfu
Seasons don't fear The Reaper.
Nor do the wind, the sun, and the rain.
We can be like they are.

I see your point on why it can be useful to separate the two, to do something in between. exec in ActiveState's port sais it will emulate it by starting a new process and then waiting for it to finish, then exiting with the same code itself. So it leaves the old program in memory!