I can't quite see the point of reading 1 char at a time, but a small variation on the server I posted in 832715 should do the job. I've only tested it on Windows, but it should run on *nix:

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

my $semSTDOUT :shared;
sub tprint{ lock $semSTDOUT; print @_; }
$|++;

my %cache;
my $Qwork = new Thread::Queue;
my $Qdone = new Thread::Queue;

my $done :shared = 0;
sub worker {
    my $tid = threads->tid;
    while( my $fno = $Qwork->dequeue ) {
        open my $client, "+<&", $fno or die $!;
        tprint "$tid: Duped $fno to $client";
        my $buffer = '';
        while( my $c = sysread( $client, $buffer, 1, length $buffer ) 
+) {
            syswrite( $client, $buffer, 1, length( $buffer ) -1 );
            while( $buffer =~ s[(^[^\n]+)\n][]g ) {
                tprint "$tid: got and echoed $1";
            }
            last if $done;
        }
        close $client;
        $Qdone->enqueue( $fno );
        tprint "$tid: $client closed";
    }
}

our $W //= 4;

my $lsn = new IO::Socket::INET(
    Listen => 5, LocalPort => '12345'
) or die "Failed to open listening port: $!\n";

my @workers = map threads->create( \&worker, \%cache ), 1 .. $W;
$SIG{ INT } = sub {
    close $lsn;
    $done = 1;
    $Qwork->enqueue( (undef) x $W );
};

while( my $client = $lsn->accept ) {
    my $fno = fileno $client;
    $cache{ $fno } = $client;
    $Qwork->enqueue( $fno );
    delete $cache{ $Qdone->dequeue } while $Qdone->pending;
}
tprint "Listener closed";
$_->join for @workers;
tprint "Workers done";
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You can stress test it locally with this (thought this might leak memory on *nix apparently):

#! perl -slw
use strict;
use threads;
use IO::Socket;

our $W //= 10;

my @t = map{
    async {
        for( 1 .. rand 1000 ) {
            my $s = new IO::Socket::INET(
                'localhost:12345'
            ) or die "Failed to connect to server: $!";
            for( 1 .. rand 1000 ) {
                print $s "Message $_";
                scalar <$s>;
            }
            close $s;
        }
    };
} 1 .. $W;

$_->join for @t;
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