$var (0) False
$var (undef) False
$var1 ('')  False
$var2 ("")  False
         $var3 (0 - as a single-quoted number) False
         $var4 (0 - as a double-quoted number) False
         $var4  (0  - as an unquoted number) True
         $var4  (0 - as a single-quoted number) True
         $var4  (0 - as a single-quoted number) has length of 1 and ASCII value of 48
$var5 (1 in single quotes) True
$var6 (1 in double quotes) True
$var7 (1 - as an unquoted number) True