No, it's available as an argument. But you shouldn't even have to look at it unless you wish to limit sending the credentials to a specific realm.

sub get_basic_credentials {
   my ($self, $realm, $uri, $isproxy) = @_;
   if ($uri->host_port() eq =~ /^(?:www\.)?example\.com:80\z/i) {
      return ($self->{user}, $self->{passwd});
   }

   return $self->SUPER::get_basic_credentials($realm, $uri, $isproxy);
}
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