As mentioned by others before, turn off autocheck.

#!/usr/bin/perl 
use strict;
use warnings;
use WWW::Mechanize;

my $browser = WWW::Mechanize->new(
              stack_depth => 0,   # Turn off memory consuming history 
              timeout     => 180, # Set timeout in seconds
              autocheck   => 0,   # Turn off autocheck, check errors m
+anually
                        );

$browser->agent_alias( 'Windows IE 6' );

my $url = 'https://foo.bar/page.html';

$browser->get($url);
if ( $browser->success( ) )  {
    print "Okay: $url. ", $browser->response->status_line(), "\n";
}
else {
    print "Error: $url. Reason: ", $browser->response->status_line(), 
+"\n";
}
[download]

Run the script with autocheck turned on (1) to reproduce your errormessage.

Hope this helps.
Thomas