Dependencies or prerequisites are an integral feature of the CPAN software repository. They define what other CPAN modules are required for a particular CPAN distribution to be built, tested, or ultimately to function, as well as optionally to improve or add functionality. To define them properly for a distribution, it is helpful to understand exactly how they will be used, and what all the different distribution files like Makefile.PL, Build.PL, META.json, and MYMETA.json are for.

sidebar: In this post, I will focus on the "requires" relationship for dependencies, which are hard dependencies that must be satisfied, but "recommends" and "suggests" relationships are also defined that indicate strongly and weakly optional dependencies respectively; CPAN installers may install these based on the options that are passed.

Most commonly and at a basic level, dependencies are defined in a (generated) file called META.json in the root of the CPAN distribution, but this may not be the complete picture. CPAN installers historically would determine what is needed at the time that the user requests to install the distribution ("install time"), and though there is now the formal concept of static prerequisites (the most common case where they are the same for every install environment), some distributions need to determine prerequisites at install time, using the original dynamic configuration process.