You don't mention it explicitly, but if you are using Thread::Pool, that is responsible for half your problems. The last time I tested that module it was badly broken and very, very slow. A quick look now and not much seems to have changed.

The other half is that you are trying to use non-blocking sockets on Windows via:

                                    Blocking => 0);
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Which unless the powers-that-be have finally gotten around to fixing it, doesn't work on windows.

Also, mixing select processing with multi-threading -- in the way you have done it -- is fraught with problems. It can be done (correctly), but is actually rarely needed.

It is hard to assess your program given that you supply bits rather than the whole thing; and omit significant details -- like the modules you are using -- but you seem to be doing things in very complicated ways.

For reference, here is a simple -- but substantially complete -- pool-based, multi-threaded echo server, that runs very well under extreme load on my Vista machine without skipping a beat:

#! 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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If you want some real help with your code -- rather than getting fobbed off with "Use this module I just found on cpan and switch to flavour-du-jour of *nix." -- you'll need to let me see your complete program (here or off-forum).