Consider our free search engine script. It is an excellent application, but it wouldn't be very helpful for others if it could display its results only in the output page format that we use for our site. Somebody else's site will probably need the output page to contain site-specific navigation links, different colors, or even a completely different layout concept. It is not uncommon to see CGI programs that contain the HTML code that is returned to the browser, inside one or more print statements in the code of the script. Clearly this is not a good idea, since it makes it at least difficult, if not nearly impossible, for a non-programmer to alter the output format. At best, a webmaster with little understanding of perl could identify the HTML code inside the program and alter some obvious items, like colours. Still, as applications grow more complex, this approach becomes even more difficult to manage. If control structures (if, while etc) are being used to generate the output, or if there are multiple pages of output composed by chunks of html that are being generated in different places in the code, even someone who is comfortable with perl could find it hard to figure out how to alter the look of the output. And this is not the only problem. The code itself becomes difficult to manage and maintain. Program code is difficult to read an maintain on its own right. Cluttering it with HTML here and there does not help the situation at all.
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