It would seem so. I run the script under strace and found that the qx op doesn't use fork syscall on my perl (Ubuntu Hardy), but clone syscall:

• A working qx...

  21696 clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETT
  +ID|SIGCHLD, child_tidptr=0x7f6015248770) = 21697
  21697 close(5)                          = 0
  21697 fcntl(6, F_SETFD, FD_CLOEXEC)     = 0
  21697 dup2(4, 1)                        = 1
  21697 close(4)                          = 0
  21697 close(3)                          = 0
  21697 rt_sigaction(SIGFPE, {SIG_DFL}, {SIG_IGN}, 8) = 0
  21697 execve("./my_program", ["./my_program"], [/* 11 vars */] <unfini
  +shed ...>
  [download]

• A non-working one...

  21742 clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETT
  +ID|SIGCHLD, child_tidptr=0x7f4143c25770) = -1 EAGAIN (Resource tempor
  +arily unavailable)
  21742 rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
  21742 rt_sigaction(SIGCHLD, NULL, {SIG_DFL}, 8) = 0
  21742 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
  21742 nanosleep({5, 0},  <unfinished ...>
  [download]

The problem is that the nanosleep syscall should not be happening.