my $Main = new Win32::GUI::Window (
     -name     => "Window",
     -title    => "Test",
     -pos      => [100, 100],
     -size     => [400, 400],
     -onResize   => \&resize,

) or die "new Window";

$Main->AddTextfield(
    -name      => "Edit",
    -pos       => [0, 0],
    -size      => [100, 100],
    -multiline => 1,
    -vscroll   => 1,
    -autovscroll => 1,
    -keepselection => 1 ,
    -readonly => 1,
);

$Main->AddTextfield(
     -name      => "Edit2",
     -pos       => [0, 0],
     -size      => [100, 24],
     -autovscroll => 1,
);

$Main->Show();

Win32::GUI::Dialog();

sub resize {
  my ($self) = @_;
  my ($width, $height) = ($self->GetClientRect())[2..3];
  $self->Edit->Resize($width+1, $height-30) if exists $self->{Edit};
  $self->Edit2->Resize($width, $width) if exists $self->{Edit2};         # Fehler!
}

  _______________________         
|                       |
|                       | 
|                       |      <------- Textfeld 1
|                       |
|                       |
|_______________________|
|_______________________|      <------- Textfeld 2