I don't know Perl, so I rewrote it in Python in an attempt to understand what's going on: [link redacted]

I'm not storing key,value pairs for elements that don't exist (not sure what the Perl code's doing), so it shouldn't be too bad in terms of space used.

I'm not entirely sure about this, but it looks more like Counting sort [1], than Pigeonhole sort.

[1]: https://en.wikipedia.org/wiki/Counting_sort