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my $head = 0;  # Can only be modified by the lone writer thread
my $tail = 0;  # Can only be modified by the lone reader thread
...
    $head = ( $head + 1 ) % @buf;
    return 1;
}

package Data::RingBuffer;

...
    $self->{head} = ( $self->{head} + 1 ) % @$buf;
    return 1;
}