If not, I can think of two methods to speed it up, both allow you to use all-lowercase filenames in your program:

1) Whenever your program starts, it checks whether there are still mixed-case filenames around (just check one file) and if yes, changes them all to lowercase (or uppercase if you prefer that). This means that when someone starts your program the first time with the downloaded data files, your program needs a little longer to start up. The next time all the files are already lowercase.

2) Generate a hash at startup that has the lowercased filename as key and the (real) mixed case filename as value. To access a file, look it up in the hash. The hash has to be generated whenever someone starts the program, but it shouldn't be a noticable delay because the program has to make just one sweep of the files on disk.

PS: Both algorithms need to do a sweep of the data files on disk. You might use File::Find to do this.