It was not the cause. Sorry for not being complete, but I really didn't think this had anything to do with it: I was testing first if the file was complete with 
flock($f, LOCK_EX) or return; #file in use

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apparently I must do a flock($f, LOCK_UN); although the transfer was in the same procedure, one line below. I don't understand why..

In reply to Re^2: found a bug in Net::FTP? by rgcosma
in thread found a bug in Net::FTP? by rgcosma

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