which does open( PUTFILE, ">$outfile" ) or threads->exit;. The threads->exit is clearly getting called, leaving the socket open.

FWIW: I have written a crap load of threaded perl code and never had occasion to use thread->exit;. It is IMO redundant and dangerous.

I would code that line as simply:

open( PUTFILE, ">$outfile" ) or return;
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That way, all the normal perl cleanup will take place before the thread function returns and the thread terminates.

Try it, it just might sort out a lot of your problems!