Actually I was about to answer just that, but then I tested it - and it turned out I was wrong.

Consider this:

$ hexdump -C foo
00000000  61 0a 30                                          |a.0|
00000003
$ perl -wle 'print "read line" while (my $x  = <>)' foo
read line
read line
$

[download]

I have a file foo here with an 'a', a newline, and a '0' - no trailing newline.

I'd expect just one line of output, because 0 is false. But why am I getting two lines? That doesn't seem to be logical to me.

So I modified my test scrpt a bit: 

$ perl -MDevel::Peek -wle 'while (my $x  = <>){ print "read line"; pri
+nt "and its true" if $x }' foo
read line
and its true
read line

[download]

So, the second line evaluates to true in the while-condition, but not in the following if-condition. Is there some kind of special magic? If yes, where is it documented?

Update ok, got it. Thank you all. I didn't realize it works for arbitrary variable names, and thought that $_ was special cased.

In reply to Re^2: Doubt on defined-ness by moritz
in thread Doubt on defined-ness by waldner

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