Is this still the best way to do it? It seems awkward that the mighty Imager wouldn't allow feeding it RGB data directly.

I chose to do it that way, because I thought it was faster than the direct way which Imager allows, which is matrix transformation. Read "perldoc Imager::Transformations, where the following matrix method is shown

 # Swap red/green channel
  $new = $img->convert(matrix=>[ [ 0, 1, 0 ],
                                                       [ 1, 0, 0 ],
                                                      [ 0, 0, 1 ] 
                                                    ]);
[download]

but, I think you will find that reversing is faster, although it will only work for swapping R and B, plus you need to invert it afterwards. But let us know, if you find otherwise.