I remember I brought up that bug, back in the days that defined was not yet implied...

If your text file ended with a "0" on a line of its own, without appended newline, that last line was ignored.

Worse: if that happened in a file in the middle of a list of files in @ARGV, then while(<>) was interrupted at the end of that file, and the rest of the files that came after it were simply ignored.

As for the Camel Book: historically, originally defined was not implied. The text in the book probably just still reflects that.