ORLY?

while( <DATA> ) {
  print defined $_ ? "Defined! " : "Undefined! ", $_;
}
__END__
Please to be not
misinforming people with
incorrect information.
[download]

The discussion on looping constructs in perlsyn clearly explains this construct. Perhaps you should try reading it?

Addendum: Not to mention the docs for $_ as well.