If open fails mysteriously (and you have no idea what the error message might be - you should inspect $! but others pointed this out already), you might try the sysopen() and sysread() family.

Here you can use a buffer with a certain length and read that number of bytes at a time into thiis buffer.

OTOH, If the problem is that you can not split on the default line endings, you might also try setting $/ to something more suitable (e.g. when the file doesn't contain newlines and you're reading a line at at time).

{
  local $/ = '>'; # you were looking for this one, right?
  open(FILE, "bigfile.big") or die $!;
  while (<FILE>) {
    # do interesting stuff
    # e.g. merely count lines...
  }
  close FILE;
}
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--
Cheers, Joe