In your last example you missed the localising, it should be:

{
   undef local $/;     # with localising
   open (FH, $filename);
   while(<FH>) {
      blah
   }
}
[download]

And then - as nobody has done this yet - I add the two definitive links to that topic, both articles from mjd found on his website Coping with Scoping and Seven useful uses of local.

-- Hofmator