Here's the kind of thing I ended up using:

#!/usr/bin/perl

use warnings;
use strict;

my $test = Foo->new();

print $test->read_only, "\n";
#$test->read_only("cheese");     # Will error

print $test->read_write, "\n";
$test->read_write("cheese");
print $test->read_write, "\n";

package Foo;

use vars '$AUTOLOAD';

sub new {
        bless {
                read_only => {
                        value    => "foo",
                        editable => 0,
                },
                read_write => {
                        value    => "bar",
                        editable => 1,
                }
        }
}; 

sub AUTOLOAD {
        no strict "refs";
        my ($self, $new_value) = @_;

        my $field = $AUTOLOAD;
        $field =~ s/.*:://;

        die "No such method $AUTOLOAD. Barfing\n"
                unless exists $self->{$field};
        if ($self->{$field}{editable}) {
                *$AUTOLOAD = sub {
                        my $self = shift;
                        $self->{$field}{value} = shift if @_;
                        $self->{$field}{value};
                };
        } else {
                *$AUTOLOAD = sub {
                        my $self = shift;
                        die "Can't modify readonly field '$field'" if 
+@_;
                        $self->{$field}{value};
                };
        }
        goto &$AUTOLOAD;

}

sub DESTROY { 1; }
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