Webmin consists of a simple web server, and a number of CGI programs which directly update system files like /etc/inetd.conf and /etc/passwd. The web server and all CGI programs are written in Perl version 5, and use no non-standard Perl modules.
Early versions were distributed as betas of what would eventually become a commercial product, but following Caldera's sponsorship of the project in 2000 all subsequent releases have been BSD licenced. Even though Caldera is no longer supporting the development of Webmin, all future releases will remain under a free licence.
Because Webmin supports the concept of modules (like PhotoShop plugins), anyone can develop and distribute their own Webmin modules for any purpose, and distribute them under any licence (such as GPL, commercial or shareware). More information about the Webmin API and writing your own modules is available.