ECHO: "special variable inheritance"
ECHO: 23, 5
WARNING: Ignoring unknown variable 'a'.
ECHO: undef
ECHO: 23, 5
ECHO: "inner variables shadows parameter"
ECHO: 5, 24
ECHO: "user-defined special variables as parameter"
ECHO: 7
ECHO: 7
ECHO: "assign only visible in children's scope"
WARNING: Ignoring unknown variable 'c'.
ECHO: undef
ECHO: 5
ECHO: "undeclared variable can still be passed and used"
ECHO: 6