#
sub server_thread {
     print "[Server]\n";
     use IO::Socket;
     use IO::Select;
     use strict;

     my $server = IO::Socket::INET::->new(Proto => 'tcp',
                                          LocalPort => 55555,
                                             Listen => 1,
                                          Reuse => 1
                                          ) or die "Server can't start
+: $!";
      
      
     my $readable_handles = new IO::Select();
...
           # It is an ordinary client socket, ready for reading.
           $buf = <$sock>;
...
[download]

Using buffered IO reads with an IO::Select server is fundamentally flawed. If any one client sends a packet that doesn't contain the right delimiter -- whether through programmer error; or because the user aborts the client mid-transmission; or because tcp decides to fragment the packet at an inappropriate point -- then your server will hang indefinitely.