If your process isn't a child of your script, no. I think it depends heavily on the operating system running, too.

The easiest way I think of on Unix systems is something like this :

use strict;
use warnings;

sub get_childpids {
    my $father=shift;
    # linux style
    my @exec=`ps -o pid --ppid $father` or die "can't run ps! $! \n";
    # SYS V style : use shell, needs cleansing
    # my @exec=`ps -eo ppid | grep $father` or die "can't run ps! $! \
+n";
    # BSD style : use shell, needs cleansing
    # my $exec=`ps aux | grep $father` or die "can't run ps! $! \n";
    shift  @exec;
    chomp @exec;
        return @exec;
}
[download]

Also you want to "get the child pid of a process running, by just passing only the parent process pid". But a parent can have many childs...

fork is not accepting arguments. This will help me when I call this function recursively to find all the child pid.So can you explain me how to pass pid as argument to fork

Maybe I wasn't clear, and I apologize for that... I wasn't suggesting fork as a function "to get a child pid of a process running, by just passing only the parent process pid". Because I can hardly make sense of such a request.

Said this, maybe you just want to get a list of all the childs of a given (parent) process. If so, then you may use Proc::ProcessTable to search amongst all processes those that are childs of the given parent. You will be particularly interested in Proc::ProcessTable::Process's ppid() method.

To put all this together is left as an exercise to the reader.

my $pid = fork();
if (!defined($pid)) {
  print "Could not fork!\n";
} elsif ($pid) {
  print " child pid was $pid\n";
} else {
  print "I am the child\n";
}
[download]