I ran into a similar issue here. My *nix Perl code, shasum (*nix), and sha256deep (Win) all returned one SHA-256 sum, but the same Perl code on Windows was returning something different here. Using "b" or "p" with addfile() did not make a difference. Then it occurred to me, the file handle itself might not have been opened correctly. The sample code above doesn't give any insight onto how the file handles were created, so I can't comment on those, but I was using "<" for the mode initially. When I switched to "< :raw", Windows started agreeing with all the other sums, no other changes needed. Just needed to be explicit about reading the binary files in a raw mode instead of whatever Perl picks on that particular platform. Apparently it chooses poorly on Windows. I hope this comment helps someone else who runs into this issue, as my hunch was more helpful to me than the Googling I did.