Calling source within a function - scoping issue

I was playing with some options and found out a fix for the problem. But I am still lacking the understanding. So I am documenting my findings for different scenarios here. Hope this helps people!

# To call within another function, reference enclosing environment!!
# Fixed my problem.
# Is there any other methods to achieve this?
CallScript<-function(ii){
  source(ii,echo = T, local=parent.frame())
}

Perform<-function(a,b){
  ii<-'~/Test/Add_a_b.R'
  CallScript(ii)
}

Perform(a=10,b=15)


# Using local=T or local=environment() options don't work in this scenario!
CallScript<-function(ii){
  source(ii,echo = T, local=T)
  #source(ii,echo = T, local=environment())
}    
Perform<-function(a,b){
  ii<-'~/Test/Add_a_b.R'
  CallScript(ii)
}    
Perform(a=10,b=15)


# I also found out that these also work.
Perform_env<-function(a,b){
  ii<-'~/Test/Add_a_b.R'
  source(ii,echo = T,local=environment())
}

Perform_loc<-function(a,b){
  ii<-'~/Test/Add_a_b.R'
  source(ii,echo = T,local=T)
}
Perform_env(a=10,b=15)
Perform_loc(a=10,b=15)


# But this doesn't work
Perform_par<-function(a,b){
  ii<-'~/Test/Add_a_b.R'
  source(ii,echo = T,local=parent.frame())
}
Perform_par(a=10,b=15)

To complicate things, here is another scenario. Except local=F, all three options work in this scenario.

# CallScript defined within `Perform` function.
Perform<-function(a,b){
  CallScript<-function(ii){
    # parent.frame works
    #source(ii,echo = T,local=parent.frame())

    # environment works
    #source(ii,echo = T,local=environment())

    # Local=T also work
    source(ii,echo = T,local=T)
  }

  ii<-'~/Test/Add_a_b.R'
  CallScript(ii)
}

Perform(a=10,b=15)

With @MrFlick's suggestion, this is also working:

CallScript<-function(ii){
  a <- get("a", parent.frame())
  b <- get("b", parent.frame())
  source(ii,echo = T, local=T)
}

Perform<-function(a,b){
  ii<-'~/Test/Add_a_b.R'
  CallScript(ii)
}

Perform(a=10,b=15)