sub submit_request {
    my ($code_ref, ...) = @_;
    my $callback_id = "$code_ref";
    $registered{ $callback_id } ||= [ 0, $code_ref ];
    ++$registered{ $callback_id }[0];
    _submit_request($callback_id, @_);
}

##</code><code>##

sub on_response {
    my $callback_id = shift;
    my $code_ref = $registered{ $callback_id }[1]
        or croak(...);
    --$registered{ $callback_id }[0]
        or delete( $registered{ $callback_id } );
    $code_ref->(@_);
}