As you may know, you can access most of the Reaper extension API via ReaScript, which lets you run Python or Perl scripts directly from Reaper, or map the scripts to action shortcuts. The scripts can call directly into Reaper API functions.
ReaScript Python should work out of the box on Macs, which come with Python installed, and should work on Windows after installing a standard Python distribution. Unfortunately Perl is not quite as user-friendly for this sort of application, so ReaScript Perl requires an extra module, called FFI. More unfortunately, FFI is not widely used, and is not readily available for OS X or for Windows x64.
The good news is, I just read this: http://use.perl.org/articles/10/05/23/1130259.shtml
which describes a currently active project to create exactly the sort of pipeline between Perl scripts and generic external libraries, that Python provides. The article, written a few weeks ago, suggests that this project will take a few months.
When Perl::Ctypes is available, Reaper will most definitely switch to using it! The transition should be invisible to Perl script writers.
So the summary is, hopefully ReaScript/Perl portability will be greatly improved by a project currently underway in the Perl world.