$ ./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