A reference to the array is stored in the global (package) variable $array_ref (which itself doesn't go out of scope).  This increments the reference count of @array, so the array doesn't "vaporize" even though it is scoped lexically to the routine.

You can use Devel::Peek to investigate the reference counts.  When you add

    Dump $array_ref;
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at the end of A(), and within B(), you'll see that the REFCNT is 2 within A(), but 1 within B()

# Dump from within A()
...
  SV = PVAV(0x138f058) at 0x13b4be0
    REFCNT = 2
...

# Dump from within B()
...
  SV = PVAV(0x138f058) at 0x13b4be0
    REFCNT = 1
...
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This is because when @array leaves its scope, the REFCNT is decremented.  If there was no reference kept to it, the REFCNT would go down to zero here, and the variable would be freed.  But due to the reference, this doesn't happen.