I know nothing about Outlook, and very little about OLE, but you read a value from an attribute

 my $body = $items->Item($itemIndex)->Body;
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modify it and then write it back to a hash element?

$items->Item($itemIndex)->{'Body'} = $body;
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Which just doesn't seem right to me. It's a pure guess, but you might try

$items->Item($itemIndex)->Body = $body;
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Assuming the Body attribute is writable through Win32::OLE, and I see no reason it shouldn't be, then that might give you the result you're after.