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 (<>).
Whether this is neccesary depends on the mail client. Gmail requires the angle brackets, Outlook does not. To make this work in the above example this should be used instead: 
$msg->attach(
        Type    => 'image/gif',
        Id      => '<ltrhead.gif>',
        Path    => 'fake_letterhead.gif',
);

[download]
The current version of MIME::Lite (3.0.1) does not add the angle brackets, but that could change in the future so be sure to check the documentation.

In reply to Sending an attachment with MIME::Lite. RFC2111 Content-Id requirements by imp
in thread Need a Mail::SendEasy example using attachments by keithneargarder

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