Why on earth would your client trust you to install your code that others have not seen or had the opportunity to evaluate, but will not trust code that many thousands have seen, trust, and use every day?

If possible, just bundle the XML::Simple code with your code and move on to a real project with clients who have realistic expectations. Since you are asking the question that you've asked, it's highly unlikely that you will be able to meet the XML::Simple functionality without a ridiculous cost overhead that the client would be foolish to ignore.

If there are reasonable objections to refusing XML::Simple, I'm all ears.