Just for completeness, here's an example of one of the approaches that ambrus already mentioned:

{
  local $^I = '';
  local @ARGV = 'c:/index.htm';
  while ( <> ) {
    s/foo/bar/g;
    print;
  }
}
[download]

I think that Tie::File rocks, but it's more firepower than the problem requires, I think.

Also, why the /m in your s///, when there is no ^ or $ in the regex? Likewise, why the /s when there is no . in the regex?