Since we’re building on Parrot, the licence of the non-original code will be the Artistic License 2.0, which is free, open-source, GPL-compatible and somewhere between a weak copyleft licence (e.g: the LGPL) and a permissive licence (e.g: the 2-clause or 3-clause BSD licences). The original code will be written under the MIT/X11 licence, which is a very permissive BSD-style licence that specifically allows sub-licensing. To avoid legal confusion, every file should contain an explicit “Licensing” notice to indicate under which license it is.