#!/usr/bin/perl

my $sid = getppid;  # for the 'ps' only

if (my $pid = fork()) {
    sleep 1; # wait somewhat until tree of children is set up
    # show related processes
    system "ps f -s $sid -o pid,ppid,pgrp,sid,cmd";

    # kill process group after timeout
    sleep 5;
    kill -9, $pid;
    wait;
    print "killed children\n";

    # show remaining processes
    system "ps f -s $sid -o pid,ppid,pgrp,sid,cmd";
    
    # do something else...
    sleep 5;
    
} elsif (defined $pid) {
    # create new process group
    setpgrp;
    # run something (consisting of several processes) that hangs
    system 'bash -c "cat; echo"';
    exit;

} else {
    die "couldn't fork";
}

__END__

Sample output:

$ ./726031.pl
  PID  PPID  PGRP   SID CMD
29171 29169 29171 29171 bash -rcfile .bashrc
  508 29171   508 29171  \_ /usr/bin/perl ./726031.pl
  509   508   509 29171      \_ /usr/bin/perl ./726031.pl
  510   509   509 29171      |   \_ bash -c cat; echo
  511   510   509 29171      |       \_ cat
  512   508   508 29171      \_ ps f -s 29171 -o pid,ppid,pgrp,sid,cmd
killed children
  PID  PPID  PGRP   SID CMD
29171 29169 29171 29171 bash -rcfile .bashrc
  508 29171   508 29171  \_ /usr/bin/perl ./726031.pl
  513   508   508 29171      \_ ps f -s 29171 -o pid,ppid,pgrp,sid,cmd