Well, it's like this. Except on very rare occasions you'll always go back and maintain your code. And in the time between now and then, you'll have learnt more stuff. Or forgotten more stuff. You'll almost certainly have forgotten the fine detail of how your program worked. In a very real sense you won't be the same you that wrote the program.
Now, if you've written the program as if you were a member of a team, then you should have used things like unambiguous variable and function names. You'll have commented your code sparsely and sensibly, being careful to document intent. You won't have used over complex syntax/algorithms simply for the sake of it. You won't have to remember the precise meaning and name of a million and one global variables.
This means that when you come to repair or extend your program you won't have to remember (or work out) as much stuff to solve your problem, you'll be able to fix things up with some confidence that doing so won't break something somewhere else. If you're really disciplined you'll even have a test suite to make sure of this. And you'll have added extra tests to confirm that the code you've added does what it's supposed to do.
All of this is stuff that you would (should?) have done if you were working in a larger team, so cultivate the good habits while you're working alone.
There's another sense in which all Perl programmers are part of a team. CPAN. Because of the generosity of other programmers you get to draw on their experience. For free. Be team spirited, if you find a bug and fix it, send patches back to the maintainer. If you've done something that's really handy, upload it to CPAN. Because you've been programming like you're a member of a team, you'll have something that is easy for others to use.
See, it all makes sense.
