package Tie::Constvar;
use Carp;
sub TIESCALAR {
    my ($class, $initval) = @_;
    my $var = $initval;
    return bless \$var => $class;
}
sub FETCH {
    my $selfref = shift;
    return $$selfref;
}
sub STORE {
    confess "cannot assign value to a constant!";
}

package main;
tie my $AVO, Tie::Constvar, 6;
print $AVO,"\n";

tie  $AVO, Tie::Constvar, 3;  #this works
print $AVO,"\n";

$AVO = 3;  #this will throw an exception!
print $AVO,"\n";