POE::Session->create(
  inline_states => {
    _start   => \&anfang,
    schleife => \&schleife
  }
);
POE::Kernel->run();
exit 0;

sub anfang {
    $poe_main_window->Label(-text => $_[HEAP]->{callback})->pack();
    $_[KERNEL]->yield("schleife");
}

sub schleife {
    $_[HEAP]->{callback} = "Hier ist der Text";        
    $_[KERNEL]->yield("schleife");
}