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  1. or download this
      DB<1> sub tst  {return undef}
  
...
FALSE
  DB<6> if (@a=tst()) {print "TRUE"} else {print "FALSE" }
FALSE
  2. or download this
      DB<9> use constant FAILED => ();
 
...
FALSE
  DB<18> if ($a=t_FAILED) {print "TRUE"} else {print "FALSE" }
FALSE
  3. or download this
           Negation of a true value by "!" or "not" returns a special fals
+e value.
       When evaluated as a string it is treated as '', but as a number
+, it is
       treated as 0.
  4. or download this
      DB<27> p !defined (FAILED)
1
  DB<28> p defined (FALSE)
1