Is this the program you are using?

If so, are you sure you aren't using the "-f|-fork" option?

My guess is, you should use my $pidline = `root-tail -f ...` but for some reason, root-tail v0.0.10 has the line for printing the PID commented out...

      pid = fork ();
      if (pid != 0)
        {
          /*printf ("forking mode: pid=%d\n", pid);*/
          exit (0);
        }
[download]

(You may want to mail the devloper and ask what gives.)

However, if you don't use -f, the code you have above seems to work just fine.