in reply to Memory Efficient Sparse Matrix Handling

You've already tried PDL's 'Sparse' module?

update: this is kind of off-the-wall, but you might also want to search cpan for RLE (run-length-encoding) - there's a bunch of image-related modules which can do rle for you.

update2: Also, see the rle() and rld() functions of pdl::slices.