In version 5, the following construct defines a default value for a function argument:
FUNCTION MyFunction as C( Input_val="My Default Value" as C )
This means that Input_val is assumed to be "My Default Value" unless the user supplies a different value when calling the function.

To make the default Input_val blank, use:
FUNCTION MyFunction as C( Input_val="" as C )

Without the default value, this would be:
FUNCTION MyFunction as C( Input_val as C )