"And... in (for example) the case of “the Web,” where, exactly, does the software begin and then cease to be “graphic?” Is Perlmonks “graphic?” Amazon.com? eBay? Perl programs all! Are they “graphic,” or not?"
No, they aren't. At least they are not good examples of visual design whether or not you separate form from function. In fact Amazon.com is a perfect example. I've been a member for years and still have a hard time finding account information. As for the form of the site, it hasn't exactly been placed in the same category as Apple or BMW for style.
So if that's what we're going to consider "graphic" simply because they chose a font and one or two colors the relevancy issue becomes more pronounced. Other languages have a culture that, at least somewhat, embrace visual design. There is no reason why Perl couldn't do the same and prosper from it as it has in from it's other accomplishments