If you're checking to see if two files match exactly, there's no need to read either into memory; you can use something like Digest::file, and compare the check sums for equality¹. In the (rather likely) event that you need to determine the actual differences between the files (as in diff or even Windows' fc, your first route should be to use a module like File::Compare.

¹ Some of the checksum algorithms will give false positives, in that identical checksums may be produced by different files. I believe "Algorithm 1" used by sum may be especially prone to this.