Bunch of questions here.

I googled API(I have no idea what it is, I just heard that it stands for application interface)..

An API in general is whatever interface the program provides so that it can be "controlled" by another program (instead of a user). Some APIs are in the form of a library of functions, others may provide some kind of network interface/protocol for instance. Many programs don't have an API at all; they just rely on button/keyboard clicks or mouse movement.

Is that the right way to approach something like that if one was interested in writing something to interace w/ 3rd party software Is perl right software to do that sort of work?

If your target program has a public API, then it's usually the most reliable and easiest way to control it. Perl's pretty good at doing that kind of thing, but the devil is as always in the details.

How does one find out what the capability of that 3rd party software and how to interface that?(I am thinking API)

Search the web. Especially the site of the manufacturer. You may need to pay for this information. An API is controlled by the target program(mer), so the info on the API is controlled by whoever controls the program. Unless you're content to rely on hacks that might break at the next update.