The OP changed his question into:

How does that script work with all the # in it?

I assume this was ment to be a reply to this post, so I'll post my answer here.

The # in s#class=section#class="Section"#sg; for example is used as a regex-delimiter, not as a comment. It is the same as s/class=section/class="Section"/sg; except that with s/// you would need to escape the / (which could make it less readable, but that does not apply in this case since it has no / in the regex)