my  $checkbox = 1;
    $_ = 'Wowza';
    print "Before the loop we see \$_ as '$_'\n";

    if( $checkbox ) {
        while( <DATA> ) {
            chomp;
            print "  We see \$_ is '$_'\n";
        }
    }

    print "After the loop we see \$_ as '$_'\n";

__DATA__
There I was, minding my own business, when ...