{
    my ( $in, $out );
    open $in, '<', $filepath or die $!;
    open $out, '>', $newpath or die $!;
    while (<$in>) {
        # process $_ which is the current line
        $_ = quux($_);
        print $out; # assumes $_ has everything to print now
    }
    close $out or die $!;
    close $in;
}