#! perl -slw
use strict;
use threads;
use Thread::Queue;

sub worker {
    my $Q = shift;

    open my $pipe, '-|', q[ perl theOtherScript.pl] or die $!;
    while( my $url = $Q->dequeue() ) {
        print $pipe $url;
    }    
}

our $THREADS //= 4;
my $Q = new Thread::Queue;

my @workers = map threads->create( \&worker, $Q ), 1 .. $THREADS;
$Q->enqueue( $url ) while ...; ## fetch urls from somewhere and Q them
$Q->enqueue( (undef) x $THREADS );
$_->join for @workers;