Probably you are serving images through your handler... a simple solution is:

sub handler
{
  my $request = shift;

  if (($request->content_type() ne 'text/html')) {
    return DECLINED;
  }
...
[download]

Or simply disable your handler via configuration:

<Location /location/of/images>
  Options ...
  SetHandler default
</Location>
[download]

There is also an error in your logic: you shouldn't send HTTP headers when you return SERVER_ERROR.

Ciao, Valerio