++ Nice summary.

Minor niggle. If it were me I would put a little bit more emphasis on lexical scoping and a little less emphasis on reference counting. When the value of a lexical variable is garbage collected is independent of whether a Perl closure is created. It's about scope - not memory usage. For example:

our $Global;

sub make_closure {
    my %lexical;
    $Global = \%lexical;
    return sub {
        my $key = shift;
        @_ ? $lexical{$key} = shift : $lexical{$key};
    };
};

{    
    # here we make a closure
    my $c = make_closure();

    # which we can use to set and get keys
    $c->(foo => 42);
    print "foo is ", $c->('foo'), "\n";
    
    # at the end of the scope the closure goes away
};

# but the referant is still around
print "Global foo is ", $Global->{foo}, "\n";
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Update: It might also be worth comparing what Perl does to languages without lexical capture like C.