As near as I can gather - it's just an innermost scope. I took the trouble to write a quickie XS library (Devel::DebugScope) to dump some of the scoping variables. You can download it here if you like: http://198.144.10.226/perl/Devel-DebugScope-0.01.tgz. I used it to instrument your code and it corroborates that CHECK and friends have an apparent lexical scope equivalent (or greater for BEGIN) than the inner scope. The key values to look at are scopestack_ix and savestack_ix (I guess).

use strict;
use warnings;
use PadWalker qw(peek_my);
use Devel::DebugScope qw(dump_scope);

CHECK { print "CHECK\n";
        Devel::DebugScope::dump_scope();
        print "\n\n"; }
CHECK { peek_my(1)->{'%foo'}->{answer} = 42 };

print "Runtime outer\n";
Devel::DebugScope::dump_scope();
print "\n\n";

{
   my %foo;
   print $foo{answer}, "\n";
   print "Runtime inner\n";
   Devel::DebugScope::dump_scope();
   print "\n\n";
};
__DATA__
CHECK
scopestack_ix:      5
savestack_ix:       21

Runtime outer
scopestack_ix:      3
savestack_ix:       10

42
Runtime inner
scopestack_ix:      5
savestack_ix:       15
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