"As a learning" exercise is about the only valid reason to eschew CPAN I can think of. As a learning exercise it is especially useful if it serves to contrast the benefit of using CPAN compared to rolling your own solution. It is even more useful if the learning exercise also involves creating even a tiny fraction of the testing that CPAN modules are subject to.

If that is the objective of your manager and you have been given time to engage in this learning exercise then you have a truly enlightened manager!