That will stop at every newline and continue again after it:

$ perl -we'$string = "abcdef\nghijk\nlmno\npqr\nst\nu";
> print map { "$_\n" } $string =~ /.{1,5}/g;'
abcde
f
ghijk
lmno
pqr
st
u
[download]

You either want the //s flag on the match or some other way of dealing with newlines.