#!/usr/bin/perl -w
use strict;
sub ps { system "ps Tf -o pid,ppid,pgrp,sid,cmd"; }
$SIG{CHLD} = 'IGNORE';
my $snoop = fork();
if ( $snoop == 0 ){
    setpgrp;
    system("sleep 20 &");
    exit;
}
ps();
sleep 1;

print  "PID:  $snoop\n";
printf "PGRP: %d\n", getpgrp($snoop);
kill -1, getpgrp($snoop);
ps();

__END__
  PID  PPID  PGRP   SID CMD
 2360  2358  2360  2360 bash -rcfile .bashrc
19529  2360 19529  2360  \_ /usr/bin/perl -w ./964597.pl
19531 19529 19529  2360      \_ ps Tf -o pid,ppid,pgrp,sid,cmd
19533     1 19530  2360 sleep 20
PID:  19530
PGRP: -1
  PID  PPID  PGRP   SID CMD
 2360  2358  2360  2360 bash -rcfile .bashrc
19529  2360 19529  2360  \_ /usr/bin/perl -w ./964597.pl
19534 19529 19529  2360      \_ ps Tf -o pid,ppid,pgrp,sid,cmd
19533     1 19530  2360 sleep 20