In breif, anywhere you can put 'content', you can put a CDATA section to contain that content

<![CDATA[ anything can go here ]]>
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If you always wrap your HTML content in a CDATA section, then you should never encounter the situation where the HTML interferes with the XML, provided you are using a properly XML complient parser.

I guess that I don't have to worry about this if I am using a module like XML::Simple?

I'm not quite sure what you mean by this?

If you mean when parsing XML?

if any XML/HTML markup in the content of the XML being parsed was wrapped in CDATA sections, then I'm fairly confident that XML::Simple would handle it, but that is a big if.

If you mean when constructing XML?

I doubt that XML::Simple will handle applying the CDATA wrapping for you. That would be very much down to you to do regardless of what you use to construct your XML.