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$ strace -etrace=select,read ./754685.pl >/dev/null
...
...
read(6, "foo\nbar\n", 4096)             = 8                         <-
+-- !!
read(9, "0\n", 4096)                    = 2
select(16, [6 9], NULL, NULL, {0, 0})   = 0 (Timeout)

        pipe $reader, $writer;
        $reader->blocking(0);
...
                $output .= $line;
            }
            ...