It is distributed under the FM Public License. What this license basically says is that you can do what you like with the source code, as long as you make the source code of your changes available to everyone else if you make your work available to anyone else.
We also ask that you understand that the FM Source Code has evolved over a long time, starting from a very hacky Perl 4 base, with a number of contributors along the way. This means that it is perhaps not as tidy as we would make it if we started from scratch now!