If you use ActivePerl on Windows, Digest::MD5 is automatically installed. Then, it seems to me that using this hash algorithm makes much sense, as it is simple and gives reliable results:

  use Digest::MD5 qw(md5 md5_hex);
  $digest = md5($data);
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Implementing your own hash means that it is probably more prone to collisions (i.e., that two different files lead to the same hash value). While MD5 has known cryptographic weaknesses, it certainly remains well suited for your task.