* The reasoning behind sub NAME { } constructs being realized early is so that you can call subroutines even if their definition is "later" in the code.

That's mostly true, but not very precise.

If a subroutine is defined after it is called, the call has to use parenthesis. If it doesn't, it's interpreted as a string literal (or as an error if strict subs are in effect).

$ perl -E 'say 1, foo; sub foo { 3  }'
1foo
$ perl -E 'sub foo { 3 }; say 1, foo'
13
$ perl -E 'say 1, foo(); sub foo { 3  }'
13
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However it the call uses parenthesis, there's no need it has to be know at compile time at all:

$ perl -E 'eval "sub foo { 3 }"; say 1, foo();'
13
[download]

But I think for this discussion it's more important to ask when the lexical pad of sub a is being built and destroyed. As long as the a is not part of the call chain, $x is in no lex pad, and sub b can't work in a meaningful way.