I don't see anything limiting the amount of concurrent connections in your code nor in PoCo::HTTP::Client's documentation. Is there something that I missed?

Trying to run your code here with a few debug statements added, the 77802 URLs are stuffed into PoCo::HTTP::Client and it consumes all the CPU. This behaviour alone seem a good reason to limit the amount of sub-tasks running at once.

Could you try to refactor your code to process N URLs at once only (10?) and see if it happens again?