#load token if previously saved
if( my $data = read_file( $tokenfile ) ) {
	$self->{token} = $data;
}

#instanciate api with token_or_undef
$self->{api} = Net::OAuth2::Client->new(
	$self->{client_id},
	$self->{client_secret},
	site                => $self->{site},
	authorize_path      => $self->{authorize_path},
	access_token_path   => $self->{access_token_path},
	access_token_method => $self->{access_token_method},
	access_token_param  => $self->{access_token_param},
	scope               => $self->{scope},
)->web_server(
	redirect_uri        => $self->{callback}
)->get_access_token(
	%{$self->{token}}
);

#...
# uses of $self->{api}->access_token() automically refreh it if needed

#and finally save the token
write_file $tokenfile => { $self->{api}->save };