#!/usr/bin/perl

use strict;
use warnings;

my $cmd = qq{nohup ./perl.pl &};

my $pid = fork;  die 'fork failed' unless defined $pid;

if ($pid) {
    print STDERR "parent pid=$$ child pid=$pid\n";
    wait;
    my $sid = getppid;
    system "ps f -s $sid -o pid,ppid,pgrp,sid,cmd";
} else {
    print STDERR "child pid=$$ \$pid=$pid\n";
    setpgrp;
    exec $cmd or die "Bad exec: $!";
}

##</code><code>##

#!/usr/bin/perl

system 'sleep 1';

##</code><code>##

$ ./813283.pl
child pid=31851 $pid=0
parent pid=31850 child pid=31851
  PID  PPID  PGRP   SID CMD
18307 18305 18307 18307 bash -rcfile .bashrc
31850 18307 31850 18307  \_ /usr/bin/perl ./813283.pl
31853 31850 31850 18307      \_ ps f -s 18307 -o pid,ppid,pgrp,sid,cmd
31852     1 31851 18307 /usr/bin/perl ./perl.pl
31854 31852 31851 18307  \_ sleep 1