This is something that I am actually using on a LARGE file and it works fine for me.
On a 30MB file, it solves the problem in about 7 seconds (linux 2.4 on a PIII 800.

perl -ne 'while (/(.{0,150})/g) {print "$1\n"}' largefile.txt
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If the remains of the file (after all the insertions) is less than 150 chars, it will be printed anyway. If you have newlines within the line, then it will come up double. But if you are sure that there are not newlines, this one will do.

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