It might help to use binmode.

Your print needs to specify what filehandle it's going to print to (print FILE pack("b*","a","b","c")).

If you want to write the bits for the characters, I think print FILE "a","b","c" would work just fine, unless I'm missing something. In any case, I think you should have a look at the pack documentation. I don't think "b*" is the format you want (but I can't tell what format you do want).