A library, as in a file of perl code with no package statement that just magically adds subs and variables to your current namespace when you use it, is generally considered to be an out-of-date approach that is supported mostly for the sake of legacy perl4 code. All new libraries should be written as modules. This is much safer in a number of ways.

Modules use other modules all the time. It is a normal and expected way to do things. It may turn out that your site is best divided into several modules with varying purposes. Most significant sites are best broken up into more than one.