You seem to be making the claim that when you have code

BEGIN {
    x();
    sub foo { }
}
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then foo() will be declared only when x() terminates.

That particular example has a flaw, but it's easy to demonstrate that the argument holds:

use Modern::Perl;

sub x { say "In x!" }

BEGIN { x() }

sub foo!bar { say 'In foo!bar!' }

BEGIN { foo!bar() }
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For extra fun, run it through B::Deparse.