Threaded code exits before all tasks are complete - multithreading

Question

I am trying to take a portion of an existing script and have it run multiple nmap scans simultaneously to increase the speed of the script.
I initially tried using fork, but it was suggested to me that I should be using threads instead as I am doing this on a Windows box. I modified a code snippet I found online and it partially works.
I am using a list of 23 IP addresses. I have been able to open 10 threads and scan the first 10 addresses, but then the code exits. Ideally, the code would open a new thread each time one exits so that there are always 10 threads running, until it reaches the remainder, in this case there would be three. Then only 3 threads would be open.
This entire code needs to be run inside a subroutine that I have in my original sequential code. I am using ping instead of the nmap command to test.
#!/usr/bin/Perl
use strict;
use threads;
my $i = 0;
my #lines;
# Define the number of threads
my $num_of_threads = 10;
# use the initThreads subroutine to create an array of threads.
my #threads = initThreads();
my #files = glob( "./ping.txt" ) or die "Can't open CMS HostInventory$!"; # Open the CMS Host Inventory csv files for parsing
foreach my $file ( #files ) {
open (CONFIG, '<', $file) or die "Can't ip360_DNS File$!";
#lines = <CONFIG>;
chomp (#lines);
}
# Loop through the array:
foreach ( #threads ) {
# Tell each thread to perform our 'doOperation()' subroutine.
$_ = threads->create(\&doOperation);
}
# This tells the main program to keep running until all threads have finished.
foreach ( #threads ) {
$_->join();
}
print "\nProgram Done!\nPress Enter to exit";
$a = <>;
####################### SUBROUTINES ############################
sub initThreads{
my #initThreads;
for ( my $i = 1; $i <= $num_of_threads; $i++ ) {
push(#initThreads, $i);
}
return #initThreads;
}
sub doOperation{
# Get the thread id. Allows each thread to be identified.
my $id = threads->tid();
my $ip = ($id - 1);
system("ping $lines[$ip] >> ./input/$lines[$ip].txt");
print "Thread $id done!\n";
# Exit the thread
threads->exit();
}