One thing you can do instead of using a tie is make your lvalue subs also have the ability to act as getters and setters. Then when you are using the object in a context where you are in control of the data you can use the lvalue assignment, but then when you want to verify data you can you use the sub in the setter mode. Example:

#!/usr/bin/perl
use strict;
package Foo;
sub new{
  my $class = shift;
  my $self = {
          bar=>""
         };
  bless $self,$class;
}

sub bar : lvalue {
  my $self = shift;
  if (@_) {
    # validate
    my $value = shift;
    if (ref($value)) {
      die "foo->bar must not be a reference!\n";
    }
    $self->{bar}=$value;
  }
  $self->{bar}
}

package main;
my $test = Foo->new;

# Use lvalue assignment
$test->bar = "Hello";
print $test->bar."\n";

# Use setter mode to verify data
$test->bar("There");
print $test->bar."\n";

# Use setter mode to verify data
$test->bar($test);
print $test->bar."\n";

__OUTPUT__
Hello
There
foo->bar must not be a reference!
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Of course this should be well documented in your pod so people don't accidently alter the lvalue.