I tested your code, and it "seemed" to work with Claws-mail. I did have to add base64 to your attachment list options. The image displayed inline, but it didn't have the cid:...... text identifier box above it, it acted like a regular attachment.

$msg->attach(Type => 'image/jpeg',
                 Id   => 'twitter-white.jpg',
                 Path => 'twitter-white.jpg',
                 Encoding => 'base64',
                 );
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