You have to set a sizer for the notebook pane, something like

$self->{sizer_browser} = Wx::BoxSizer->new(wxVERTICAL);
$self->{sizer_browser} ->Add( $browser, 1, wxEXPAND, 0);
$self->{notebook_1_pane_1}->SetSizer($self->{sizer_browser});

    $self->SetSizer($self->{sizer_1});
    $self->{sizer_1}->Fit($self);
    $self->Layout();
[download]

Its easiest to lay it out in wxglade