My (very limited) experience of working with PDL suggests that if you need to manipulate the values in a piddle individually, rather than applying each operation to the entire piddle as a whole, then you should export the piddle, en-masse, to a perl array first. It saves huge amounts of time,

And if part of the reason you are using piddles is to save memory, and that effectively prevents you from exporting the whole piddle to a perl array, then export it in large chunks 1000 or 10,000 at a time and overwrite the array with the next chunk.

Of course, you should make sure that your operation cannot be done using a PDL function before resorting to exporting.

One example was finding the max and min values of a million doubles. Accessing the values individually from Perl and comparing was more than an order of magnitude slower than exporting the whole lot and performing the operation in Perl; but using the minmax function was (from memory, I don't currently have a working PDL installation) 2 orders of magnitude faster than exporting.

The biggest problem was actually finding the appropriate functions, which are often barely mentioned and weirdly named.