Typically, the scope of a variable is only as wide as the routine its defined in. Thus, the life of your variables is only valid as long as each routine is executing. As soon as the sub/function is exited, the variables are released. A variable defined globally (not within a sub/function, like args in the second example)), lives and is visible throughout the entire plugin.
In the example you cited, I'd assume the author was referring the the value of the variable being deleted rather than the variable itself.
In your code, you don't really need to sent a parameter. You could simplely obtain the plrid again by subtracting out the BASE_TASK_ID
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