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Very nice explanation. Perhaps I should fill in the details leading up to my original post:
While rereading Effective Perl Programming I noticed that it warned against constructs such as: while($line = <FILE>) and recommended using while(defined($line = <FILE>)) instead. The book is several years old ('97 I think??) and I knew that this particular issue had been dealt with in later versions of perl. I had also noticed a "real" erratum in the book and decided to track down the books errata page so I could keep track of such things as I read the book.
Well, it turns out that I couldn't find a list of errata... The best I could come up with was this empty page which looks lilke it should contain a list of errata but it doesnt. A one line '* this issue has been fixed in later versions' would have ended the story right there.
I then went looking for the documentation covering this automatic definedness feature. The trivial 'perldoc -f while' doesn't work, because while isn't a function. 'perldoc -q while | grep defined' didn't turn up anything, and searching for 'while and defined' over at perldoc.com or Super Search turned up way too much. I checked a few online docs around here, including perlop to no avail. It turns out that perlop is so long, the Monastery's copy is split in two... the info I was looking for was on the second page of perlop which would have ended my search had I found it.
Fine, the docs had failed me... I'd just have to figure it out on my own. B::Deparse to the rescue! After deparsing a dozen constructs, I was more confused than ever... <FILEHANDLE> seemed well behaved, I could make a few conclusions about how it would play with definedness and while. However, globbing didn't seem to play by the same rules. Sometimes it added the defined() sometimes it didn't. Hindsight shows that there was really only one case that didn't play nicely, but at the time it confused the hell out of me.
Frustrated, I posted to perlmonks, and the rest is covered quite nicely above.
P.S. Thanks robin, you made my day.