I get the same results as JavaFan.

By overriding WWW::Mechanize, I determined that I don't get any redirect at all:

package MyWWWMechanize;
use strict;
use warnings;
use WWW::Mechanize;
our @ISA = qw( WWW::Mechanize );

#---------------------------------------------------------------------
+-----
# redirect_ok - log redirect messages
#---------------------------------------------------------------------
+-----
sub redirect_ok
{
    my ($self, $prospective_request, $response) = @_;

    # Call parent to do all the boring work.
    my $ok = $self->SUPER::redirect_ok($prospective_request, $response
+);

    # Log it
    print "Redirect From: ".$response->as_string()."\n";
    print "Redirect To:   ".$prospective_request->as_string()."\n";
    print "Redirect is ".($ok ? "Allowed" : "DENIED")."\n";

    return $ok;
}
1;

package main;
my $url='https://www.platts.com/Login.aspx?';
#my $mechanize = WWW::Mechanize->new(autocheck => 1);
my $mechanize = MyWWWMechanize->new(autocheck => 1);
$mechanize->cookie_jar(HTTP::Cookies->new);
$mechanize->agent('Mozilla/5.0 (X11; U; Linux i686; en-GB; rv:1.8.1) G
+ecko/20061010 (IKDhPmJcdw) Firefox/2.0');
my $response = $mechanize->get( $url );
my $html = $mechanize->content;
print "$html  \n";
[download]

Are you sure you're getting this redirect on a GET and not a POST? Have you messed around with requests_redirectable?