Here I tie $_ to a Complainer package that can warn/carp/croak any time something tries to assign to it.

#!perl -l

package Complainer;
use Tie::Scalar;
use base Tie::StdScalar;
use Carp;

sub STORE {
   # Warn out any status info you want here.
   carp "Tried to assign to \$_";
}

sub FETCH { $_ }

package main;

tie $_, Complainer;

$_ = 1;

local $_;
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Update: Added the fetch method so that reading $_ actually works. not sure why I have to do that, Tie::StdScalar should handle that for me... but this was written in a hurry.