cpan install($_) doesn't work in loop, but normal lexical var does

[tphyahoo]

use strict;
use warnings;
use CPAN;
for ( 'Catalyst::Plugin::Authentication' ) { install ($_)} #doesn't wo
+rk
#for my $mod ( 'Catalyst::Plugin::Authentication' ) { install ($mod)} 
+#works

The error I get with the $_ variant happens at the write makefile step:

... blah blah blah...

Removing previously used /home/thartman/.cpan/build/Catalyst-Plugin-Au
+thentication-0.09

  CPAN.pm: Going to build N/NU/NUFFIN/Catalyst-Plugin-Authentication-0
+.09.tar.gz

Checking if your kit is complete...
Looks good
Writing Makefile for Catalyst::Plugin::Authentication
Modification of a read-only value attempted at /home/thartman/perlroot
+/perl/lib/5.8.8/CPAN.pm line 4676.

I don't understand why my $var works, but $_ from the loop doesn't. I thought $_ was automatically lexically scoped, and could be relied on to do the same thing as specifying my $var before the for list. What gives?

By the way, to test this, I had my perl root directory tarred up. Before running the script I reverted my perl root directory by

thartman@nbartertest:~/nbarter$ cat ~/perlroot-revert
#!/bin/bash
rm -rf perlroot
tar -xzvf perlroot.tar.gz

Then I ran perl -e 'use Catalyst::Plugin::Authentication'

to confirm that this produced an error because of the missing module, and then I would run the script to see if things installed ok or not.

Also, there's nothing special about Catalyst::Plugin::Authentication. This error seems to be produced regardless of what module I try to install.

Blessings to the monastery.