DB<1> sub tst  {return undef}
  
  DB<2> if ($a=tst()) {print "TRUE"} else {print "FALSE" }
FALSE
  DB<3> if (@a=tst()) {print "TRUE"} else {print "FALSE" }
TRUE
  DB<4> sub tst  { return (); }
  
  DB<5> if ($a=tst()) {print "TRUE"} else {print "FALSE" }
FALSE
  DB<6> if (@a=tst()) {print "TRUE"} else {print "FALSE" }
FALSE

##</code><code>##

  DB<9> use constant FAILED => ();
 
  DB<10> use constant FALSE => !1;
 
  DB<11> use constant TRUE => 1;
 
  DB<12> sub t_FALSE  { return FALSE; }
 
  DB<13> sub t_TRUE  { return TRUE; }
 
  DB<14> sub t_FAILED  { return FAILED; }
 
  DB<15> if (@a=t_FAILED) {print "TRUE"} else {print "FALSE" }
FALSE
  DB<16> if (@a=t_FALSE) {print "TRUE"} else {print "FALSE" }
TRUE
  DB<17> if ($a=t_FALSE) {print "TRUE"} else {print "FALSE" }
FALSE
  DB<18> if ($a=t_FAILED) {print "TRUE"} else {print "FALSE" }
FALSE

##</code><code>##

       Negation of a true value by "!" or "not" returns a special false value.
       When evaluated as a string it is treated as '', but as a number, it is
       treated as 0.

##</code><code>##

  DB<27> p !defined (FAILED)
1
  DB<28> p defined (FALSE)
1