I don't have an answer to the main part of your question, as I haven't used UDP, but I have an answer for something fundamental that might be missed.

You don't need to fork or thread to use TCP efficiently. It is sometimes a good way to do it, but here is an example of another way. If you like it, read the manual pages for the packages useed. Notice in particular IO::Select.

use IO::Socket;
use IO::Select;
use Tie::RefHash; #there are also lots of slower ways to keep track of
+ the connections ;)
use POSIX; #for usefull constants

my %connections;
my $port = 5000;
tie %connections, qw( Tie::RefHash ); # so we can use refs as hash key
+s

my $server = IO::Socket::INET->new(
                   Listen    => SOMAXCONN,
                   LocalPort => $port,
                   Reuse     => 1,
                   Proto     => 'tcp',
                  )
  or die "can't open connection: $!";
$server->blocking( 0 );

my $select = IO::Select->new( $server );

while('eternity'){
    foreach my $connection ( $select->can_read(60) ) {
    if ( $connection == $server ) {
        # new connection waiting
        my $client = $connection->accept;
        $client->blocking( 0 );
        $select->add( $client );
        $connections{$client} = time; # I like to know how long a user
+ has been connected, you could also set this to anything; we're just 
+using the key for fast lookup.
    }
    elsif ( exists $connections{$connection} ) {
        # user sending us data
        my $length = $connection->sysread(my $data,POSIX::BUFSIZ);
        if (defined $length && length $data) {
            print "got '$data' from ",$connection->peerhost,':',$conne
+ction->peerport,"\n";
        }
        else {
            # No data, probably EOF or an error, so close
            $select->remove( $connection );
            delete $connections{$connection};
        }
    }
}
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