while (<STDIN>) { print; }      # tests for definedness
print while <STDIN>;            # tests for definedness

##</code><code>##

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

##</code><code>##

% 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]