The following lines are equivalent:

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

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.

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Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

I noticed that the behavior was consistant for filehandle reads, which is why I found the behavior of globbing perplexing...

The following lines are equivalent to each other:

while (<STDIN>) { print; }      # tests for definedness
print while <STDIN>;            # tests for definedness
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But these two are not equivalent to each other:

while ($_ = <*>) { print }      # tests for definedness
print while ($_ = <*>);         # tests for truthfulness
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Update -- Here is an example in action:

% mkdir newdir
% cd newdir
% touch 0 1 2
% ls
0  1  2

% perl -le 'while ($_ = <*>) { print }'
0
1
2

% perl -le 'print while ($_ = <*>)'     
[note nothing is printed here]
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-Blake