my %values = (
        0      => 0,
        zero   => '0',
        undef  => undef,
        empty  => '',
        1      => 1,
        one    => '1',
        string => 'string',
);

printf ("%6s : %i : %i :: %i :: %i\n", $_, ($values{$_}?1:0),
         ( $values{$_} == 0),
         (($values{$_} or $values{$_} == 0)?1:0),
         (($values{$_} or $values{$_} eq 0)?1:0))
     foreach sort keys %values;

##</code><code>##

     0 : 0 : 1 :: 1 :: 1
     1 : 1 : 0 :: 1 :: 1
 empty : 0 : 1 :: 1 :: 0
   one : 1 : 0 :: 1 :: 1
string : 1 : 1 :: 1 :: 1
 undef : 0 : 1 :: 1 :: 0
  zero : 0 : 1 :: 1 :: 1