I ran your code and there was no indication in the output that any errors occurred.

200
200
Back 1 OK
200
Back 2 OK
200
Back 1 OK
200
Back 2 OK
[download]

I improved your test so we can actually see the problem:

#!perl

use strict;
use warnings;

use WWW::Mechanize qw( );

my $mech = WWW::Mechanize->new();

#$mech->proxy(['http', 'ftp'], 'http://localhost:8080/');

my $url = 'http://www.vitoco.cl/test-ref';
#my $url = 'http://www.vitoco.cl/test-ref/index2.html';

$mech->get($url)->is_success() or die;

@ARGV or die;

for my $img ($mech->images) {
  if ($ARGV[0]) {
    $mech->head($img->url_abs())->is_success() or die;
    $mech->back() or die;
  }

  my $response = $mech->get($img->url_abs());
  print($mech->status(), "\n");
  print( ( $response->redirects() )[0]->request()->referer(), "\n");
  $mech->back() or die;

  print("\n");
}
[download]

$ perl a.pl 0
200
http://www.vitoco.cl/test-ref

200
http://www.vitoco.cl/test-ref


$ perl a.pl 1
200
http://www.vitoco.cl/test-ref/img1

200
http://www.vitoco.cl/test-ref/img1
[download]

When I upgraded WWW::Mechanize, the back returned false after a head.

$ perl a.pl 0
200
http://www.vitoco.cl/test-ref

200
http://www.vitoco.cl/test-ref


$ perl a.pl 1
Died at a.pl line 22.
[download]

So yes, WWW::Mechanize is buggy.

    if ( $request->method eq 'GET' || $request->method eq 'POST' ) {
        $self->_push_page_stack();
    }
[download]

should be changed to

    $self->_push_page_stack();
[download]

Please file a bug report.