Your quoting is broken. You are doing

>perl -le"print length unpack 'A', \'n';"
1
[download]

which is the same as

>perl -le"print length unpack 'A', 'S';"
1
[download]

If you fix the quoting, you get the correct result.

>perl -le"print length unpack 'A', qq{\n};"
0
[download]

pack says "When unpacking, «A» strips trailing whitespace and nulls".