broquaint,
I have been going through my old nodes looking for interesting stuff. I aplogize for a reply almost 3 years later.

There is a slight difference between my statement of
"has to read the entire file to index the newlines..."
and your statement of
"It does indeed read the whole file into memory,.."

As best as I understand Tie::File, it doesn't actually read the file into memory. It reads the entire file yes, but only enough stays in memory as is needed while it searches for input record separators for indexing.