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',
);
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.