You're not checking whether or not your nickname is in use. Try this:

use Data::Dumper;
use POE::Kernel;
use POE::Session;
use POE::Component::IRC;
use strict;

sub start {
  my ($kernel, $session) = @_[KERNEL, SESSION];

  $kernel->post( 'test', 'connect', {
            Debug    => 1,
            Nick     => 'Bot',
            Server   => 'irc.psam.se',
            Port     => '6667',
            Username => 'BotV6-Poe',
            Ircname  => 'ModHelp', }
           )and print("connecting\n");
$kernel->post( 'test', 'register','all')and print("registered\n");
}

sub stop {
  my ($kernel) = $_[KERNEL];

  $kernel->call( 'test', 'quit', 'Neenios on ice!' );
}

sub on_connect {
   my ($kernel) = $_[KERNEL];
   print("in on_Connect\n");
   $kernel->post( 'test', 'join', '#Alex' )and print("Joined\n");

}

sub on_msg {
    my $kernel = @_[KERNEL];
    my @args = @_[ ARG0 .. $#_ ];
    my $list=join(" . ",@args);
    $kernel->post('test','privmsg','Isa',@_[ARG0]);
    }

sub conn {
my ($server) = $_[ARG0];
print("connected to $server\n");

}

sub in_use {
 my $kernel = @_[KERNEL];
 print "Nickname is currently in use\n";
 $kernel->post( 'test','nick','Mr' . int rand 200);
}

POE::Component::IRC->new('test');
POE::Session->create(inline_states =>{
                                   irc_connected=>\&conn, 
                                   _start=>\&start,
                                   _stop=>\&stop,
                                   irc_001=>\&on_connect,
                                   irc_msg=>\&on_msg,
                                   # ERR_NICKNAMEINUSE => 433
                                   irc_433=>\&in_use 
                                   },);


$poe_kernel->run();
[download]

Hope this helps.