Closures clarification

props has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks, I know a lexical resides in its enclosed block, this is a definite closure

my $total_size =0;
find{ sub $total_size += -s if -f}, '.');
print $total_size, "\n";
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why this is not a closure though ?

use warnings;
use strict;
{
my $a;
}
$a=1
print $a;
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in my eyes my $a is visible outside the {} many thanks