muppet on the gtk-perl mailinglist found the solution ;-)
here's his message (well, at least the solution part ;) ):

Is the image data actually raw, packed, 24-bit pixels?  If it's in a  
compressed format, you'll have to do incremental loading with a  
PixbufLoader.

/me pokes google for EXIF thumbnail format

According to http://park2.wakwak.com/~tsuruzoh/Computer/Digicams/exif-
+ 
e.html#ExifThumbs the thumbnail will be JPEG or TIFF, not raw bits.

Do this, instead:

   my $loader = Gtk2::Gdk::PixbufLoader->new;
   $loader->write ($data);
   $loader->close ();
   my $pixbuf = $loader->get_pixbuf ();

(That code is from memory, check for yourself to make sure i'm not  
telling lies.)
[download]

It works like a charm now ;-)