(*I have an idea about the cause of the slowness in 5.10; I'll need to think of a way to verify it.)
I came across this article a little while back. It might help explain the speed difference. I certain have seen a difference running a Perl application between 5.1x and 5.22.

How We Spent Two Days Making Perl Faster

Hope you find it interesting!

In reply to Re^6: PDL: Looking for efficient way to extract sub-images, by finding bounding boxes of "objects" (7000x faster) by dbuckhal
in thread PDL: Looking for efficient way to extract sub-images, by finding bounding boxes of "objects" by vr

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