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

##</code><code>##

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

##</code><code>##

    The following lines are equivalent:

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

    This also behaves similarly, but avoids $_ :

        while (my $line = <STDIN>) { print $line }    

    In these loop constructs, the assigned value (whether assignment
    is automatic or explicit) is then tested to see whether it is
    defined.  The defined test avoids problems where line has a string
    value that would be treated as false by Perl, for example a "" or
    a "0" with no trailing newline.  If you really mean for such values
    to terminate the loop, they should be tested for explicitly:

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