What's the most defensive way to loop through lines in a file with Perl?

I usually loop through lines in a file using the following code:

open my $fh, '<', $file or die "Could not open file $file for reading: $!\n";
while ( my $line = <$fh> ) {
  ...
}

However, in answering another question, Evan Carroll edited my answer, changing my while statement to:

while ( defined( my $line = <$fh> ) ) {
  ...
}

His rationale was that if you have a line that's 0 (it'd have to be the last line, else it would have a carriage return) then your while would exit prematurely if you used my statement ($line would be set to "0", and the return value from the assignment would thus also be "0" which gets evaluated to false). If you check for defined-ness, then you don't run into this problem. It makes perfect sense.

So I tried it. I created a textfile whose last line is 0 with no carriage return on it. I ran it through my loop and the loop did not exit prematurely.

I then thought, "Aha, maybe the value isn't actually 0, maybe there's something else there that's screwing things up!" So I used Dump() from Devel::Peek and this is what it gave me:

SV = PV(0x635088) at 0x92f0e8
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0X962600 "0"\0
  CUR = 1
  LEN = 80

That seems to tell me that the value is actually the string "0", as I get a similar result if I call Dump() on a scalar I've explicitly set to "0" (the only difference is in the LEN field -- from the file LEN is 80, whereas from the scalar LEN is 8).

So what's the deal? Why doesn't my while()