package My::Plugin::AnyEvent;

...

sub execute_async {
    my ($self,$command, $complete) = @_;
    $complete ||= AnyEvent->condvar;
    # Send the command to Firefox
    $self->handle->push_write($command);
    # Read the response as soon as it becomes available
    $self->handle->push_read(line => sub {
        $complete->send($_[0]); # send the data to whoever waits
    });
    $complete
};

sub execute {
    my ($self,$command) = @_;
    # synchronously execute the command
    $self->execute_async($command)->recv
};