I'm trying to put together a simple "Ken Burns" style slideshow using SDL, however when I try and move the image inside of the app the portion of the image that was cropped because the app window is smaller than the image doesn't get drawn and the portion of the app window I'm moving away from doesn't update -- these leads to a neat streaking effect that I'd like to get rid of. Here's the proof of concept I've been playing around with:

#!/usr/bin/perl
use SDL;
use SDL::Event;
use SDL::App;
use SDL::Rect;
use SDL::Surface;

use warnings;
use strict;

my $app = new SDL::App (
    -title    => 'Ken Burns Slideshow',
    -width    => 640,
    -height    => 480,
    -depth    => 32,
);

my $frame = SDL::Surface->new( -name => '/path/to/testimage.jpg' );

my $frame_rect = SDL::Rect->new(
    -height    => $frame->height(),
    -width    => $frame->width(),
    -x    => 0,
    -y    => 0,
);

my $dest_rect = SDL::Rect->new(
    -height    => $frame->height(),
    -width    => $frame->width(),
    -x    => 0,
    -y    => 0,
);

$frame->blit( $frame_rect, $app, $dest_rect );
$app->update( $dest_rect );

for my $x ( 1 .. 200 ) {
    $dest_rect->x( $x );
    $dest_rect->y( $x );
    $frame->blit( $frame_rect, $app, $dest_rect );
    $app->update( $dest_rect );
    $app->sync;
    $app->delay( 10 );
}
[download]

What am I missing? Thanks!!