No it hasn't. You seem to be confusing Foo with the anonymous sub

1. BEGIN { ... } is compiled.
   1. my $x = 12345; is compiled.
   2. *Foo = sub(){ $x }; is compiled.
2. BEGIN { ... } is executed.
   1. my $x = 12345; is executed.
   2. *Foo = sub(){ $x }; is executed. ⇐ &Foo is declared and defined here.
3. print Foo() is compiled. ⇐ &Foo is known to be constant.
4. [ Execution phase starts ]
5. print 12345 is executed.

vs

1. { ... } is compiled.
   1. my $x = 12345; is compiled.
   2. *Foo = sub(){ $x }; is compiled.
2. print Foo() is compiled. ⇐ &Foo is implicitly declared and called as a plain sub here.
3. [ Execution phase starts ]
4. { ... } is executed.
   1. my $x = 12345; is executed.
   2. *Foo = sub(){ $x }; is executed. ⇐ &Foo is defined here.
5. print &Foo() is executed.

In fact, the prototype is always completely ignored if the subroutine call is compiled before the prototype is added.

*Foo = sub($) { print "$_[0]\n" };
sub Bar($) { print "$_[0]\n" }

my @array = qw( a b c );
Foo(@array);  # a
Bar(@array);  # 3
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Maybe due to another bugfix? Who knows...

Anyway, IMHO the docs should be updated to be clearer in this respect.