or if there were a lex_local that could just be dropped in for lexicals as local can for globals.

First of all local is the wrong approach, because local just manipulates already declared pack-vars, so under strict you would be forced to predeclare your variables with "our" (or "my") in the outer scope. (look at first our in the following code) (see update)

I would love to have a lexical local in P5 BUT not with the extra magic of declaring missing variables your assuming.

What you really want is aliasing like $_[ ] variables do.

I know about *, although it doesn't seem to do much for lexicals, but what do you mean by pack-vars?

Aliasing with package-variables

use strict;
use warnings;
$\="\n";
 
my $lamb = sub { 
  our ($d,$e); # local doesn't declare!!!
  local (*d,*e)=\(@_); 
  $d += $e ;
};
 
our $d="D";
our $u=10; 
$lamb->($u,5); 
print $u;  #> 15
print $d;  #> D
[download]

However, I don't know how to improve.

from your /msg:

What can I do to improve? I try to phrase my questions as (1) an informal description of what I want; (2) imaginary code that'll do what I want; (3) descriptions of what I've tried, and what I don't want to do (ideally, with explanations).

some ideas:

1. Post working code which exactly does what you want, but in a "unpretty" way. e.g. with
   • $_[0] or
   • lex-refs $$x or
   • aliased pack-vars *x ...
2. Express what you want to avoid in the working example (uglyness, doubelsigils, pack-vars,...)

3. Post a test program which proofs every functionality you want to have covered, like
   • aliasing,
   • locality..
e.g. with Test:More like in this post
4. Write a codegenerator using eval which covers all edge cases e.g.
   • with or without use strict;,
   • with or without use warnings;,
   • variables known/unknown in outer scope,
   • outerscope vars are lex or pack
5. Make clear which moduls are acceptable to be used...
   • None
   • Just CORE
   • CPAN w/o XS
   • CPAN
   • any

UPDATE: as JavaFan pointed out here you are not forced to use our with full-qualified varnames, so this works

my $lamb = sub { 
  #  our ($d,$e);
  local (*::d,*::e)=\(@_); 
  $::d += $::e ;
};
[download]

but I don't know if this meets your criteria of being more elegant than using $_[0] += $_[1] and furthermore it's only a pack-var thing, in P5 lex-vars have no namespaces...