Defunct ("zombie") processes arise when the respective process has terminated, but its (still alive) parent hasn't reaped the child's status by waiting for it.

When you modify check.pl as follows 

#!/usr/bin/perl

use POSIX ":sys_wait_h";

print "process id: $$\n";
my $pid = fork;
my $FH;
unless($pid){
    setpgrp;
    exec('./script');
}
print "Child process id is: $pid\n";
for (1..5) {
    kill -9, $pid if $_ == 4;  # kill children in the 4th round
    system "ps Tf -o etime,pid,pgrp,stat,cmd";
    sleep(3);
    waitpid(-1, WNOHANG);
}

[download]

you'll get something like 

$ ./check.pl 
process id: 4609
Child process id is: 4610
    ELAPSED   PID  PGRP STAT CMD
      06:17  4569  4569 Ss   bash
      00:00  4609  4609 S+    \_ /usr/bin/perl ./check.pl
      00:00  4610  4610 R         \_ /usr/bin/perl ./script
      00:00  4611  4609 R+        \_ ps Tf -o etime,pid,pgrp,stat,cmd
I am running
I am running
I am child and I'am running
I am running
    ELAPSED   PID  PGRP STAT CMD
      06:20  4569  4569 Ss   bash
      00:03  4609  4609 S+    \_ /usr/bin/perl ./check.pl
      00:03  4610  4610 R         \_ /usr/bin/perl ./script
      00:02  4612  4610 S         |   \_ /usr/bin/perl ./script
      00:01  4613  4610 S         |   \_ /usr/bin/perl ./script
      00:00  4614  4609 R+        \_ ps Tf -o etime,pid,pgrp,stat,cmd
I am child and I'am running
I am running
I am child and I'am running
I am child and I'am running
I am running
I am child and I'am running
I am child and I'am running
I am running
    ELAPSED   PID  PGRP STAT CMD
      06:23  4569  4569 Ss   bash
      00:06  4609  4609 S+    \_ /usr/bin/perl ./check.pl
      00:06  4610  4610 S         \_ /usr/bin/perl ./script
      00:05  4612  4610 S         |   \_ /usr/bin/perl ./script
      00:04  4613  4610 S         |   \_ /usr/bin/perl ./script
      00:03  4615  4610 S         |   \_ /usr/bin/perl ./script
      00:02  4616  4610 S         |   \_ /usr/bin/perl ./script
      00:01  4617  4610 S         |   \_ /usr/bin/perl ./script
      00:00  4618  4610 S         |   \_ /usr/bin/perl ./script
      00:00  4619  4609 R+        \_ ps Tf -o etime,pid,pgrp,stat,cmd
I am child and I'am running
I am child and I'am running
I am child and I'am running
I am running
I am child and I'am running
I am child and I'am running
I am child and I'am running
I am running
I am child and I'am running
I am child and I'am running
I am child and I'am running
I am child and I'am running
I am running
    ELAPSED   PID  PGRP STAT CMD
      06:26  4569  4569 Ss   bash
      00:09  4609  4609 S+    \_ /usr/bin/perl ./check.pl
      00:09  4610  4610 Z         \_ [script] <defunct>
      00:00  4623  4609 R+        \_ ps Tf -o etime,pid,pgrp,stat,cmd
    ELAPSED   PID  PGRP STAT CMD
      06:29  4569  4569 Ss   bash
      00:12  4609  4609 S+    \_ /usr/bin/perl ./check.pl
      00:00  4624  4609 R+        \_ ps Tf -o etime,pid,pgrp,stat,cmd

[download]

As you can see in the next-to-last ps output, all children have been killed, but the script process is defunct, because its parent hasn't yet gotten around to reaping it (due to how I deliberately wrote the loop).  Three secs later, in the last ps output, the process is no longer there, because waitpid has been called in the meantime.

In reply to Re^7: Obtain the child process id in perl by almut
in thread Obtain the child process id in perl by Anonymous Monk

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