I've finally got something that works. It's very sketchy and p6-newbish, but it does the job. Obviously there is a LOT more I need to learn, but I'm content that I've got a proof-of-concept done. The following code was written/tested on a FreeBSD system.

anonymonk above was right; :buf, Buf was the key here.

se v6;
use experimental :pack;

my $fn = 'in.txt';
my $outfile = 'out.txt';

# write a new file with win32 endings

my $fh = open $fn, :w;
$fh.print("ab\r\ndef\r\n");
$fh.close;

# re-open the file for reading

$fh = open $fn, :bin;

my $eol_found = False;
my Str $recsep = '';

# read one byte at a time, as we have no way to tell end of 
# line from EOF, and we don't want to slurp the whole thing

while $fh.read(1) -> $buf {
    my $hex = $buf.unpack("H*");
    if $hex ~~ /(0d|0a)/ {
        $eol_found = True;
        $recsep = $recsep ~ $hex;
        next;
    }
    if $eol_found {
        if $hex !~~ /(0d|0a)/ {
            last;
        }
    }
}

$fh.close;

my %recseps = (
    '0d0a' => "\r\n",
    '0d'   => "\r",
    '0a'   => "\n",
);

my $nl = %recseps<<$recsep>>;

# open a new file for writing with our new recsep

$fh = open $outfile, :w;
$fh.print('a' ~ $nl);
$fh.close;

# re-read the outfile and see if our saved recsep
# was written

$fh = open $outfile, :bin;

my $buf = $fh.read(1000);
say $buf;

__END__
Buf[uint8]:0x<61 0d 0a>
[download]

Now I'll go back to reading the docs and practicing what I'm learning. I'm sure within a single day I can clean that code up a whole heck of a lot :)

After I'm more familiar with Perl6, I'll go back to attempting to convert my module.

Thanks for all the feedback here!