runrig,
I just spent the better part of an hour playing with Want. Indeed, it can do what I was suggesting albeit very clumsily. 
#!/usr/bin/perl
use strict;
use warnings;
use Want;

my %obj;
sub cool :lvalue {
    my $key = shift;
    my %ok = map { $_ => 1 } 1..7;
    my $junk;

    if ( want( 'LVALUE ASSIGN' ) ) {
        if ( ! $ok{ $key } ) {
            warn "$key is not valid";
            lnoreturn;
        }
        else {
            my ($rval) = want( 'ASSIGN' );
            if ( ! $ok{ $rval } ) {
                warn "$rval as rval is not valid";
                lnoreturn;
            }
            $obj{ $key } = $rval;
        }
        lnoreturn;
    }
    elsif ( want( 'RVALUE' ) ) {
        if ( ! $ok{ $key } ) {
            warn "$key is not valid";
            rreturn $junk;
        }
        else {
            rreturn $obj{ $key };
        }
    }
    else {
      warn "Houston, we have a problem";
    }
    return;
}
cool( 1 ) = 3;
print cool( 1 ), "\n";
cool( 5 ) = 42;
cool( 9 ) = 3;
print "$_ : $obj{$_}\n" for keys %obj;
__END__
3
42 as rval is not valid at foo.pl line 20.
9 is not valid at foo.pl line 14.
1 : 3

[download]
At first I thought it must be a source filter then I noticed that it used XS and stopped looking. It is neat but not practical IMO.

Cheers - L~R

In reply to Re^2: Experimenting with Lvalue Subs by Limbic~Region
in thread Experimenting with Lvalue Subs by Limbic~Region

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.