Thank you all, monks, for your insights regarding my question. I'm currently in the process of reading "Programming Perl". Today I discovered the book explicitly explains <> and $_. It also confirms your answers, especially ysth's. The first part of `Line Input (Angle) Operator' in `Input Operators' in Chapter 2 covers the subject thoroughly. A short quote:

If and only if the line input operator is the only thing inside the conditional of a while loop, the value is automatically assigned to the special variable $_.

After that some useful examples follow.