Overriding a module subroutine from a class subroutine

I have a class which has an ability to define a credentials.

$po = PoService->new()->set_basicauth("jack", "secret");

The thing is, in order to do that, it has to redefine a module subroutine. So, I did it this way:

sub set_basicauth {
  my ($self, $creds_username, $creds_password) = @_;

  sub SOAP::Transport::HTTP::Client::get_basic_credentials {
    return $creds_username => $creds_password;
  }

  return $self;
}

But, when I run the code, it tells me a message:

Variable "$creds_username" will not stay shared at /opt/PoService.pm line 53.
Variable "$creds_password" will not stay shared at /opt/PoService.pm line 53.

What did I do wrong here?