How to multithread seeing if a webpage exists in perl?
===

I'm writing a perl script that takes in a list of urls and checks to see if they exist (note that I only care if they exist. I DON'T care what their contents are). Here's the important part of the program.

    use LWP::Simple qw($ua head);

    if (head($url))
    {
        $numberAlive ++;
    }
    else
    {
        $numberDead ++;
    }

Right now the program works fine; however, I want it to run faster, thus I'm considering making it multithreaded.  I assume that the slow part of my program is contacting the server for each url; therefore, I'm looking for a way in which I can send out requests to the urls of other webpages on my list while I'm waiting for the first response.  How can I do this?  As far as I can tell, the head routine doesn't have a callback that can get called once the server has responded.
