The problem here is that print is not atomic ... Here's a script that provokes it:

Hm. And the fix is sooo complicated:

use strict;
use threads;
use threads::shared;

my $sem :shared;

open my $fh, '>', 'outfile' or die $!;
my $th = 0;
my @threads = map {
    $th++;
    async( sub { sleep(1); for(1 .. 30_000) { lock $sem; print $fh "Th
+read $th\n" } } );
} (1 .. 500);
$_->join foreach @threads;
close $fh;
[download]

A whole 3 lines.

Regarding modules to facilitate the implementation of said state machine, one I found easy to use (actually the only one I've ever used in production code) is POE::Component::Client::UserAgentPOE::Component::Client::HTTP. (edited, it's been a while but the name didn't sound quite right) POE is rather heavyweight though (not that it mattered much here) so AnyEvent::Curl::Multi might be worth a look too.

Okay, so where's the code? How about you run what you brung?

Betcha don't!

And if you do, betcha it takes you 10 times longer to write; requires 10 times as much (user) code; requires 20 times as many support modules that accumulate to be 30 times as much actual non-core code to trust the authors of and require outside support for when it goes wrong; and finally, runs slower and less efficiently than the 5 minute-to-write, 30 line threaded script above.