TCP provides you with a stream of bytes. It's your job to convert that into a stream of messages. To do that, you need to define what constitutes a message. If the messages aren't fixed-length, the transmitted data itself must provide information allowing you to isolate each message.

For example, each message could end with a sentinel value, such as as a newline. If so, you'd use something like the following:

my $buf = '';
for (;;) {
   my $rv = sysread($sock, $buf, 64*1024, length($buf));
   die $! if !defined($rv);
   last   if !$rv;

   while ($buf =~ s/^([^\n]*\n)//) {
      process_msg("$1");
   }
}

die("Partial message") if length($buf);
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Or you could prefix each message with the length of the message. If so, you'd use something like the following:

my $buf = '';
my $want;
for (;;) {
   my $rv = sysread($sock, $buf, 64*1024, length($buf));
   die $! if !defined($rv);
   last   if !$rv;

   for (;;) {
      if ($want) {
         last if length($buf) < $want;
         process_msg(substr($buf, 0, $want, ''));
         $want = 0;
      } else {
         last if length($buf) < 4;
         $want = unpack('N', substr($buf, 0, 4, ''));
      }
   }
}

die("Partial message") if $want || length($buf);
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