sub do_something()
{
   my $obj = shift @_;
   my $crit = new Foo::Lock( $obj );
   $crit->lock();

   # now do something in the critical section ...
   $obj->yada_yada();
   return unless $obj->valid();

   # now do some more stuff...
   $obj->yada_yada();

   return( $result );
}