open(MFILE, ">$completeadd") || die "file open failed: $!\n";
flock(MFILE, LOCK_EX) || die "Lock failed: $!";
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Yup, that's the culprit. You are zeroing the file before obtaining the flock, thus possibly stomping on someone else's read or write.

Either use File::CounterFile, or read my columns that use flock for working examples.