I just noticed your SO has some answers. Perhaps those give you the info you need and what follows is redundant but maybe not, and I'd already written this, so:

• Read Sinan Unar's Christmas blog about newline handling for a wrap up of the last round of changes in Perl 6 handling of newlines.

• If that doesn't provide all the answers you need, consider digging deeper (in particular, search for 'newline' in jnthn's blog posts about NFG to understand some key constraints on how Perl 6 must handle newlines due to NFG (due to Unicode), and/or maybe skim the #perl6 daily logs that mention both user nanis (Sinan) and newlines) and/or visiting the #perl6 or #perl6-dev freenode IRC channels.

ETA: This bit from Sinan's post:

We don't do any such translation when using .encode/.decode, and of course when reading/writing Bufs to files, providing an escape hatch from translation if needed.

reads to me like a very direct claim that there's a way to read and write files without translating newlines.

Hth.