To avoid the problem with the realm I used an URI with the user info included. Next subroutine works perfect:

#-----------------------
# Subroutine check_link
#-----------------------
sub check_link {
  my $url  = $_[0];

  my $user  = 'Admin';
  my $pass  = '********';
  my $UriInfo = 'http://'.$user.':'.$pass.'@';

  $url =~ s/http:\/\//$UriInfo/;

  my $ua  = LWP::UserAgent->new;
  my $res = $ua->head($url);

  if ($res->is_error) {
   return '<FONT COLOR="#FF0000">X</FONT>';
  } else {
   return '<FONT COLOR="#008800">V</FONT>';
  }
}
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Thank you all for the very meaningful contributions.