OK, I think it should look like this:

use strict;
use Wx qw/wxSOLID/;

package MGV;
use base 'Wx::App';

sub OnInit 
{
  my $self = shift;
    
   $self->{frame} = Wx::Frame->new( undef, -1, 'title', [500,500], [50
+0,500] );
    
       
 Wx::Event::EVT_PAINT($self,\&paint);

sub paint{
    my ($self,$event) = @_;
      my $dc = Wx::PaintDC->new( $self->{frame} );

      
        $dc->DrawRectangle( 100,100, 200,200 );
      
    }
  $self->{frame}->Show( 1 );
}

package main;

MGV->new()->MainLoop();
[download]

Thanks for posting