Update: To clarify, by "you never clear $data", I mean you rely on sysread to clear $data, and it's entirely reasonable to fit to not do so when it detects there's nothing to read.

Your right I made an assumption about sysread clearing the data in the example.
Clearing $data and $rc inbetween had no effect.
Yes, both the blocking short example and the non-blocking full version are behaving the same way on windows as the sysread result is greater than zero.
I checked the simple client and larger version on HP-UX with the same behavior. I also added enough code to the example to send an ack back so I don't think it is a resend.

That's not how it should behave, and I cannot replicate your results (ActivePerl 5.8.8 on WinXP).

sysread blocks until the server sends data, and which point the data is returned in $data, overwriting what's already there.

The only reason you'd get the same data repeatedly was if the server was sending it repeatedly.

For testing, I used

#!/usr/local/bin/perl

# client.pl

use strict;
use Socket;
use Time::HiRes qw( sleep );
use IO::Handle;
my $prot = getprotobyname('tcp');
my $host = inet_aton("localhost");
socket(SOCK, AF_INET, SOCK_STREAM, $prot) or die("socket failed to sta
+rt");
my $dest_addr = sockaddr_in(5003, $host);
connect(SOCK, $dest_addr) or die("Connect failed");
my $data = 'aaaaaaaaaaaaaaaaaaaaaaaa';
while (1) {
   print STDERR "do a read\n";
   sysread(SOCK, $data, 1024);           # reads the data on the serve
+rs socket
                                         # but when loop around again 
+the data is still there as server 
                                         # has not written anything ne
+w
   print STDERR "data was $data\n";
   sleep(0.2);
}
close SOCK;
[download]

#!/usr/bin/perl

# server.pl

use strict;
use warnings;

use IO::Socket::INET;

my $sock = IO::Socket::INET->new(
   Proto     => 'tcp',
   LocalPort => 5003,
   Listen    => 1,
) or die;

my $client = $sock->accept()
   or die;

for (;;) {
   my $x = sprintf( '%04d', rand(1000) );
   print( "$x\n" );
   print $client ( $x ) or last;
   sleep(1);
}
[download]

By the way,

use IO::Handle;
my $prot = getprotobyname('tcp');
my $host = inet_aton("localhost");
socket(SOCK, AF_INET, SOCK_STREAM, $prot) or die("socket failed to sta
+rt");
my $dest_addr = sockaddr_in(5003, $host);
connect(SOCK, $dest_addr) or die("Connect failed");
[download]

is much more readable as

use IO::Socket::INET;
my $sock = IO::Socket::INET->new( 
   Proto     => 'tcp',
   PeerAddr  => 'localhost',
   PeerPort  => 5003,
) or die $!;
[download]