After a little testing, it seems to work well. Hard to say for sure with these race conditions. I did make a change to the _retrieve sub to lock the semaphore file even if the data file does not exist:

sub _retrieve {
    my $data_file = 'data/plugin_portfolio';
    my $lock = $data_file . '.lck';
    open (LOCK, "> $lock") or die "Can't open lock file";
    flock(LOCK, LOCK_EX);

    return 0 if !-f $data_file;
    retrieve $data_file;
}
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