#! perl -slw
use strict;
use IO::Socket;
use threads qw[ yield ];
use threads::shared;
use Thread::Queue;

sub worker {
    my $Q = shift;
    while( my $fnum = $Q->dequeue ) {
        open my $client, "+<&$fnum" or die $!;
        while( <$client> ) {
            print $client $_;
            print $_;
        }
        close $client;
    }
}

our $WORKERS ||= 5;

my $Q = new Thread::Queue;
my @workers = map threads->create( \&worker, $Q ), 1 .. $WORKERS;

my $server = IO::Socket::INET->new(
    LocalHost=>'localhost:54321', Listen=>5, Reuse=> 1
) or die $^E;

while( my $client = $server->accept ) {
    my $fno = fileno( $client );
    $Q->enqueue( $fno );
    yield while $Q->pending; ## wait until a worker dups the socket
    close $client;           ## Now we can safely close it.
}