my $pid;
defined($pid = fork()) or die "unable to fork: $!\n";
if ($pid == 0) { # child
  exec(...);   # exec R script here
  die "unable to exec: $!\n";
}
# parent continues here

##</code><code>##

  exec("R", "and", "some", "args");  # $pid will be the R command
  exec("R and some args  > out"); # $pid will be that of a shell