Roy Johnson,
Great idea. I would just expand it a little more. I would have a structure that looked like this:

my %mp3 = (
    byname => {};
    bymd5  => {};    
);
[download]

Again, as you stated each key in the secondary level hash would be an array reference to a list of matching files. The difference here is that you will also get a list of duplicate file names in different directories that may not be the same song. This can cause problems when you try to merge the directories. I would suggest the following modules:

• MD5
• File::Find or File::Find::Rule
• File::Basename

Cheers - L~R