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while (my $line = <STDIN>) {
  &do_something_with($line);
}

while (defined(my $line = <STDIN>)) {
  &do_something_with($line);
}

    The following lines are equivalent:

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

        while (($_ = <STDIN>) ne '0') { ... }
        while (<STDIN>) { last unless $_; ... }