#!/usr/bin/perl

package ConstScalar;

sub TIESCALAR {
   my $class = shift; my $val = shift;
   return bless \$val, $class
}

sub STORE { }
sub FETCH { my $self = shift; return $$self; }

package main;

my $pi;
tie $pi, 'ConstScalar', 3.14159265358979323846;
print "$pi\n";
$pi = 5;                # no effect
print "$pi\n";