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rfc2111 specifies:
Note: in Internet mail messages, the addr-spec in a Content-ID [MIME] or Message-ID [822] header are enclosed in angle brackets (<>). 
my $msg = MIME::Lite->new(
    Type    =>'multipart/related'
);
$msg->attach(
    Type => 'text/html',
    Data => qq{<body>Image: <img src="cid:myimage.gif"></body>},
);
$msg->attach(
    Type => 'image/gif',
    Id   => '<myimage.gif>',
    Path => '/path/to/somefile.gif',
);
$msg->send();

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