I need to fool vim to think buffer starts at column n

Hi list,

I'd like to stop vim from modifying the first n columns of a buffer, no
matter what I do in it.

Consider the following data, where the number isn't vims internal line
numbers, but actual data:

50 ./bin/
51 ./cgi/
52 ./dev/
53 ./devlaleh/
54 ./emu/
55 ./etc/

Now, I'd want to perform any action in this buffer, for example a simple:

:%s/ /_/

and I need vim to *think* that the buffer starts at column 9.

I know that I can solve this with mappings and functions, but this isn't
an option in this case.

How would I go about solving this please?

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