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.

In reply to Re^3: <STDIN> not initializing $_ by carol
in thread <STDIN> not initializing $_ by carol

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