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
[download]

But these two are not equivalent to each other:

while ($_ = <*>) { print }      # tests for definedness
print while ($_ = <*>);         # tests for truthfulness
[download]

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]
[download]

-Blake