How to insert a new line character after a fixed number of characters in a file
How about something like this? Change 20 is the number of characters before the newline, and temp.text is the file to replace in..
sed -e "s/.\{20\}/&\n/g" < temp.txt
Let N be a shell variable representing the count of characters after which you want a newline. If you want to continue the count accross lines:
perl -0xff -pe 's/(.{'$N'})/$1\n/sg' input
If you want to restart the count for each line, omit the -0xff argument.