This is really extremely nice. Some questions:
I noticed that after adding a folder to the library, it simply adds all the filepaths in that folder to lib.pl. It doesn't cache any of the metadata. This means that it has to rescan everytime. I'm wondering what thoughts went into that design decision. On the one hand, it's very easy to manage, since it's just a list of files. On the other hand, for large collections, startup time can be considerable. I have 109GiB of music, but with a top of the line SSD, it only took ten or fifteen seconds to scan the whole thing, which isn't bad.
On a similar note, it appears that because it adds all the files to the lib.pl file, and not the top level directory I added, it doesn't have any support for noticing when new files are added to my ~/music folder. This means I have to re-add ~/music when I add files to it. I guess that's not too bad. And I assume it won't add duplicates or anything if I re-add the root level directory. But what about when I remove items? Will it be smart enough to detect that they're not there? (An algorithm like: "I just traversed a directory for which there is an existing file entry in lib.pl, but during this traversal I didn't see it. I should therefore remove it.")