i think you are looking for your result in the wrong variable...

You are printing out your request as a string.
Probably what you want is the result, ie the returned page. The variable you're looking for in that case is:

$res->content;
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Here's a modified version of your code, but be warned that i can't test it here at work (no perl, and i can't install it).

#!/usr/lib/perl -w

use strict;

use LWP::UserAgent;
use Crypt::SSLeay;
use HTTP::Request::Common;
use HTTP::Cookies;
use LWP::Simple;
use LWP::Debug qw(+);

my $ua = LWP::UserAgent->new;

$ua->cookie_jar(HTTP::Cookies->new(file => "cookie_jar", autosave => 1
+));
   $ua->timeout(300);
   
my $req = POST 'https://adwords.google.com/select/main',
             ['login.userid'       => 'test@test.com', 
              'login.password'     => 'testabc',
              'cmd'             => 'LoginValidation',
              'login'            => 'Login'
             ];

    my $res = $ua->request($req);
 
    unless ($res->is_success) {
        print "Login Failed: : ". $res->status_line .  "\n";
        return 0;
    }

#  These lines are unnecessary
#  my $b = $req->as_string;
#  my $req = '';

print "\n\n\nThe 1st result is:\n$res->content\n";
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Or, if you really prefer to abstract, you could say:

my $b = $res->content;
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