To add to Joost's comments: An Application Program Interface (API) is supplied by the 3rd party and should be documented with the product. Many APIs are designed to be driven from C, being a fairly portable, generic, and widely known language. We can create a Perl interface by writing a variety of C code (known as XS) between Perl and the 3rd-party product. This code will use the API of the product and also the Perl API, see perlapi.

Once the code is written (if in a language like C) then the 3rd-party Run-time libraries are linked with the program. This means that you must have those libraries in order to build it, and that might mean buying a license.

Some examples: Windows has a C API, known as Win32. UNIX also has a C API, documented in UNIX man pages (section 2). Microsoft Office, on the other hand, only has the OLE API, it does not have a C procedural interface (which is a real pain). Most commercial database products have C APIs, and indirectly this enables interaces to languages like PHP, Python, and good ole' Perl.