Primarily libraries are the former -- units of code you call from your code as part of the same program. Many but not all of them use an object interface with modules, while others use subroutine calls.

See perlmod for information on Perl modules, which is how most libraries for use in Perl are presented. See perlboot for a beginner's guide to object-oriented Perl. There's plenty to learn here on PM, too. You can search for different topics that have already been discussed or post questions specific to a module to SoPW.