What is the following code supposed to do?

qr/\.(rtf&ftp)$/
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Where in perlre is that way of working documented?

Personally, I wouldn't try to cram the whole logic into File::Find::Rule. I would partition the work as follows:

1. Find all files
2. Remove all file names that don't end in .rtf or .ftp
3. Sort the filenames
4. For each .ftp file, look if the file following it has an identical name, except for the last four letters.