BEGIN {
  if ($condition) {
    use Foo;
  }
}

##</code><code>##

BEGIN {
  if ($condition) {
    BEGIN {
      require Foo;
      Foo::import();
    }
  }
}

##</code><code>##

print Bar::new();
print Bar::old();

package Bar;

sub new {
  sub old {
    return "I'm old";
  }
  return "I'm new";
}

1;