now how can i ignore the image part, find the nested subparts?

The following will deal with all the parts of an entity, skipping any parts that are mime type 'image/gif':

foreach my $part ($entity->parts_DFS) {
    my $type = $part->mime_type;
    next if($type eq 'image/gif');
    print "Here's a part: $type\n";
    # do what you want with the part here
}
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