Having a look at the forest with all those trees in the way...

Functionally, pop does 2 things:
   1] removes the last element from an array and
   2] returns that last element (for you to do things with)

Passing a literal array rather than a named variable means that the balance of the array is lost to the great bit-bucket in the sky immediately.
That means that the effect 1] above is lost.
That means that all you are really doing is 2] - getting the last entry in a list.

and that is very easily:

$x=[1,3,5,7,9]->[-1]
[download]

or

print [1,3,5,7,9]->[-1]
[download]

(Well, hard-coded like this, you may as well just grab the '9' and be done, but let's pretend this is auto-generated in an eval or something.)

HTH