1. the variable must exist (=> is there a key in the hash-reference of 
      variables with this name?) and be defined (so it may not be assigned undef);
   2. the value of the variable must not be empty (e.g. may not be a string 
      with no content);
   3. the value of the variable may not be zero (as in the integer 0);

In any other case, the expression will be evaluated to false.