#!/usr/bin/perl

# call.pl

use warnings;
use strict;

my $lock_file = 'lock.lck';

print "$0: Calling subtask script...\n";

system("/usr/bin/perl busy.pl &");

my $start = 0;

while(1){

    # on first loop, sleep two seconds before checking
    # for the lock file

    sleep(2) if ! $start;
    $start++;

    if (-e $lock_file){
        print "$0: Subtask still running...\n";
        sleep(1);
        next;
    }
    print "$0: Done!\n";
    exit;
}

##</code><code>##

#!/usr/bin/perl

# busy.pl

use warnings;
use strict;

my $lock_file = 'lock.lck';

open my $lock, '>', $lock_file
  or die "Can't grab a lock file: $!";

print "$0: Starting subtask...\n";

sleep(5);

print "$0: Ending subtask...\n";

close $lock;
unlink $lock_file;

##</code><code>##

$ perl call.pl

call.pl: Calling subtask script...
busy.pl: Starting subtask...
call.pl: Subtask still running...
call.pl: Subtask still running...
call.pl: Subtask still running...
call.pl: Subtask still running...
busy.pl: Ending subtask...
call.pl: Done!