You'll need to fork the R script yourself like this:

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
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Note that depending on how you call exec, $pid will either be the pid of the R command or the pid of a shell that perl creates to invoke the R command. Examples:

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