This looks like a very simple DOM manipulation: delete nodes from the DOM. I can do that in Firefox's developer tools. I am sure any DOM manipulator can do that. Specifically the Mojo::DOM suggested by marto should also be able to do it - but I have not used it before. In short: parse your html and convert it to a DOM, which is a Tree of html-tag nodes. Locate the node by xpath or other exotic selector. Zap the node and/or its children. Work at as high level as you can with this one because the spec will continually change and change and it will come to bite you.
Edit: I am not sure if the process of HTML -> DOM -> manipulate -> HTML will retain exactly the white spaces from the original HTML between tags as it seems you want to keep them given the test cases you provided.
Add: a regex "is simpler" but it isn't.
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