Using Parallel::ForkManager in foreach loop - arrays

Question

I am just learning Perl as a fourth language.
My wish is to use Parallel::ForkManager to speed up a foreach loop using an array whose members are taken from a text file.
Basically I am testing a .txt file of URLs, and wish to make it so that it will test multiple members of the array at once, not one at a time (five at a time in this instance) and without spamming the same URL inadvertently DoSing it.
Would something like this do the trick?
$limit = new Parallel::ForkManager(5);
foreach (#lines) {
$limit->start and next;
$lines = $_;
... do processing here ...
$limit->finish;
}
or would it be the equivalent of running that loop 5 times making a small multithreaded DoS script?

Answer 1

It isn't too clear from the documentation, but
A call to start will block in the parent process until there are fewer children running than the limit specified. Then it will return the (non-zero) child PID in the parent, and zero in the child
A child process can see all the data in the parent process as it was when the start was called. The data is presumably copy-on-write, as the child may modify it but the changes aren't reflected in any other process's workspace
The $pm->start and next idiom may seem a little obscure. Essentially it skips the rest of the loop if the start method returns a true value. I prefer something like my $pid = $fm->start; next if $pid; or the if construct in the code below. Both do the same thing, but I think more legibly
I recommend that you experiment with this simpler application, which uses a cache of five child threads to print the numbers from zero to nine.
use strict;
use warnings;
use Parallel::ForkManager;
STDOUT->autoflush;
my $fm = Parallel::ForkManager->new(5);
for my $i (0 .. 9) {
my $pid = $fm->start;
if ($pid == 0) {
print "$i\n";
sleep 2;
$fm->finish;
}
}

Answer 2

To test, use a safe local process like print or write to avoid spamming the URL's. Here's a working snippet from a program I wrote that uses the fork manager.
my $pm=new Parallel::ForkManager(20);
foreach $add (#adds){
$pm->start and next;
#if email is invalid move on
if (!defined(Email::Valid::Loose->address($add))){
writeaddr(*BADADDR, $add); #address is bad
$pm->finish;
}
#if email is valid get domain name
$is_valid = Email::Valid::Loose->address($add);
if ($is_valid =~ m/\#(.*)$/) {
$host = $1;
}
$is_valid="";
# perform dsn lookup to check domain
#mx=mx($resolver, $host);
if (#mx) {
writeaddr(*GOODADDR, $add); #address is good
}else{
writeaddr(*BADADDR, $add); #address is bad
}
$pm->finish;
}