...

  # when the "New" menu item is activated, a panel should render
  EVT_MENU($self, $ID_MAKE_PANEL, \&_make_panel );
  EVT_MENU($self, $ID_EXIT, sub { $self->Close(1) });

  $self;
}

sub _make_panel {
    my ( $self ) = @_;

    return if exists $self->{'panel'};

    use Wx qw( wxVERTICAL wxGROW );

    my $panel = Wx::Panel->new( $self,          # parent
                                -1,             # id
                                [ -1, -1 ],     # position
                                [ 200, 200 ] ); # size
    my $sizer = Wx::BoxSizer->new(wxVERTICAL);

    $sizer->Add( $panel, 1, wxGROW );

    $self->SetSizer($sizer);
    $sizer->SetSizeHints($self);

    $self->{'panel'} = $panel;
}

...