use strict;
package ConstantScalar;
use Carp;
require Tie::Scalar;
@ConstantScalar::ISA = 'Tie::StdScalar';

sub STORE 
{
 confess "illegal store into constant";
}

# call with a list of refs to scalars
sub makeConstant
{
 while (my $ref = shift)
 {
  (warn "nonscalar ref", next) if (ref($ref) ne 'SCALAR');
  tie $$ref, 'ConstantScalar', $$ref;
 }
}

package main;

my ($A, $B, $C, $D) = (1, 2, 3, 4);
my $E = 5;
print "$A $B $C $E\n";

ConstantScalar::makeConstant(\($A, $B, $C, $D));
$E = 123;
$A = 4;  # illegal store into constant
$B = 5;

print "$A $B $C $E\n";