Sometimes someone will say "No modules" and there is a very good reason for it that they have no control over. But usually there isn't. What I think is particularly bad are the people who say that they don't want to use a module because they think that modules are slow. If getting the program ready quickly and making it robust is less important to you than raw performance, well that is a battle that has been fought from assembler to C to Perl, to Perl with modules - guess which side always wins in the end?

So someone shows up and says "no modules". Should we not make sure that the person really does understand what they are giving up?

OTOH we should - even while checking that - be sure not to be impolite because they really might be unable to do anything about the restriction.

Now I know I'm gona get voted down for this but there are times when I just don't want to use modules Yes, Thats right I just choose not use them. Why? well to learn more about how things work. I'm not saying modules are bad but many times I make programs not for the purpose of getting something in particular done but to learn HOW to do things. There is (IMHO) a certain wisdom in knowing more about how to do something than just calling a given module.
As a rule I won't use a module unless I know at least in general how it's doing what it's doing.
There is one other issue with modules. If I write a piece of code I know how it works, but using a module means reading many man pages which I find extremely annoying. as such while a module may have more capabilities because I don't know how to use it to it's fullest my own code works better. For example, I can't stand OO, my mind doesn't work that way, so if you point me to an OO module it's not going to do me much good.
Again, I'm not saying modules are bad, I use them frequently. but they should not be view as an all purpose no matter what use this instead of your own code.
of course this is all IMHO.

If you are truly concerned with using what you understand then I humbly submit that you should stop using regular expressions. Why? Because I assure you that you have no concept how it really works. (Did you know that it can find fixed substrings *faster* than you can possibly scan the original string?)

Besides which I can guarantee that your brain doesn't work like Ilya's does. (Ilya is the guy responsible for most of the RE engine.)

What? You are unconvinced? You tell me that regular expressions are just too darned useful to give up? Let me think about that for a second...

Well I don't like OO very much either. I am quite vocal on that in fact. But OO is a useful tool. Whether or not I find it fun, I would be foolish to not learn how to use it and use it effectively. And indeed I have. Likewise no matter how much fun it might be for me to be tilly against the world (I can outcode them all!), in the real world that is a really bad way to do things.

So yes. Try to learn how things really work. But when it comes down to getting stuff done, don't let your research stop you from putting out a working solution. And no matter how much you learn from rolling your own, be aware that your code, with less testing, outside advice, etc, is going to be less mature, capable, secure and so on than existing standard solutions are. Yes, this is even going to be true most of the time when you are a hotshot programmer who really is better than the rest. (Which most of us are not going to be.)

Besides which, going it alone is more work. So do as Larry says, be lazy and learn to use CPAN. (Yes, even the modules from CPAN which are internally OO. Do you have to care if it works?)