my $socket_io_handlers = $mech->repl->expr('variable_that_handles_the_callbacks');

$socket_io_handlers->{on_chat} = sub {
    print "socket.io: Got " . Dumper \@_;
};

##</code><code>##

my $create_tee = $mech->repl->declare(<<'JS');
    function (next_handler, perl_handler) {
        next_handler.apply(arguments);
        perl_handler.apply(arguments);
    };
JS
my $old_handler = $socket_io_handler->{ on_chat };
my $new_handler = $create_tee->( $old_handler, sub { print "socket.io: " . Dumper \@_ });
$socket_io_handler->{ on_chat } = $new_handler;