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

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

% mkdir newdir
% cd newdir
...

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