A few rants:
1) HTML is structural, which can infer presentation, but it is not presentational itself (that's what CSS is for)
2) Being really picky here, but <p> marks the start of the paragraph, and always has done.

Right, to go back to the original question; In xml (and therefore in xhtml), you cannot has empty elements, so <img> is invalid. <img></img> is not (although completely pointless). As a shorthand elements that contain no data can be abbreviated to <img /> which is what is being spewed out by CGI.pm in this instance.

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RatArsed