Well, tables (as furniture) aren't permanent as well; as soon as it starts to show age or wear, it will be replaced, just as perl scripts that don't scale well or keep up with current technologies.
Even though software is less permanent than other creations (furniture, art, music, etc.) there's another difference--you can continually imporove software. To me, that's even more appealing than permanence. If a carpenter creates a table and looks at it decades from now, there still may be things he/she would change. Furniture styles change, tables get sturdier, etc. (Yeah, you get the drift. The analogy is starting to wear down).
I love working on a piece of software, finishing it, looking at it proudly, then start hacking at it again a while later to make it better. The sky's the limit, and sometimes that's frustrating if you just focus on what you could do, but overall it forces you to continually work at mastering your craft and producing better and better quality work. And you also have the benefit of building on top of an existing work, not always having to start from scratch.