There appears to be a lot of confusion in this thread. I hope this node will help to clear things up.

In older versions of Perl, this code:

while (my $line = <STDIN>) {
  &do_something_with($line);
}
[download]

would exit the while loop early under one condition; when the last line of input is '0', with no trailing newline. The solution to this problem was to add a defined() test around the assignment:

while (defined(my $line = <STDIN>)) {
  &do_something_with($line);
}
[download]

However, as of perl5.005, the defined() test is performed automatically! Thus, the above two snippets of code would be equivalent.

Here's what the documentation from perl5.6.0 has to say, in a snippet from perlop :

    The following lines are equivalent:

        while (defined($_ = <STDIN>)) { print; }
        while ($_ = <STDIN>) { print; }
        while (<STDIN>) { print; }
        for (;<STDIN>;) { print; }
        print while defined($_ = <STDIN>);
        print while ($_ = <STDIN>);
        print while <STDIN>;

    This also behaves similarly, but avoids $_ :

        while (my $line = <STDIN>) { print $line }    

    In these loop constructs, the assigned value (whether assignment
    is automatic or explicit) is then tested to see whether it is
    defined.  The defined test avoids problems where line has a string
    value that would be treated as false by Perl, for example a "" or
    a "0" with no trailing newline.  If you really mean for such value
+s
    to terminate the loop, they should be tested for explicitly:

        while (($_ = <STDIN>) ne '0') { ... }
        while (<STDIN>) { last unless $_; ... }
[download]

(A similar explanation can be found in the 5.005 docs, but I found this one from 5.6.0 to be clearer.)

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