Rather than saying "foreach reads everything into memory first," it's more accurate to say that the <FH> operator behaves differently in scalar and list contexts. Scalar context returns the next line of the file - list context returns all of them. When to call by reference, and when not, is a little complicated, but a good rule of thumb is that if you're not absolutely sure what you're doing, you definitely shouldn't call by reference. If you are absolutely sure, you maybe still shouldn't.