I have not ever used or developed VOIP software, but I will try to help you get passed the initial obstacles of starting your project with general advice.

As others said, there are numerous open-source packages for what you want to do. Start with installing their "server" on your computer. Most likely, using Linux will make these installations easier. I had very hard time keeping old Mac hardware, which limits OS version, up-to-date with new applications. Can you afford a Raspberry? Or, I had success installing modern linux on 15-year old laptops. There are strip-down versions of Linux specifically for running on old hardware. There are resources on how to install voip servers (mumble for example) on rapsberry (https://pimylifeup.com/raspberry-pi-mumble-server/) or Linux (https://wiki.mumble.info/wiki/Installing_Mumble).

Once you are successful with the server, experiment with connecting a ready-made, open-source voip client to it. Each voip software will offer some kind of C/C++/Java API for creating your own client. So, if you are successful with connecting with 3rd-party client, it is now time to create your own client. Nothing fancy, avoid GUI. Make a CLI program, following tutorials, on how to place a call, for example. All 3 languages C/C++/Java are supported by the fabulous Inline module and this will be the easiest way to "bind" your client with Perl.

There is also linphone, which is what it's called a softphone, a very good idea. Linphone offers a SIP server (installation).

If you have your own SIP server then you can use Net::SIP as has already been suggested to contact it. Linphone also offers an API for writing apps for it.

You have quite a few options, it seems.

If I may suggest: use linux whenever possible, it will make your life easier and will empower you and your compatriots. And btw I can think of something worse than a "3rd-world country": a developed country with people of "3rd-world" mentality.

bw, bliako