I haven't used it, but the install part should be easy. You may have Perl bindings for GTK2 already installed. If not, there's a package for your system. Just apt-get and you have it. Then you probably need a tutorial, like http://gtk2-perl.sourceforge.net/doc/gtk2-perl-tut/
If your whole application is written in C++ you need to re-design the architecture to make the C++ and Perl parts work together.
Also, I'm moving this thread to Programming.