在计算机程序设计中，回调函数，或简称回调（Callback），是指通过函数参数传递到其它代码的，某一块可执行代码的引用。这一设计允许了底层代码调用在高层定义的子程序。

没啥不好理解的呀，就是向函数的参数为   一个函数的引用呀。。

[root@wx03 ~]# cat a2.pl

use AE;

   use AnyEvent;

##定义watch

    sub run {

    my $code=shift;

    my $t = AnyEvent->timer(

        after    => 0,

        interval => 1,

        cb       => $code,

    );

my $cv = AnyEvent->condvar;

  $cv->recv;

        };

sub test {

print  "11111111111\n";

};

run(sub {test()});

[root@wx03 ~]# perl a2.pl

11111111111

11111111111

[root@wx03 ~]# cat a2.pl

use AE;

   use AnyEvent;

##定义watch

    sub run {

    my $code=shift;

    my $t = AnyEvent->timer(

        after    => 0,

        interval => 1,

        cb       => $code,

    );

my $cv = AnyEvent->condvar;

  $cv->recv;

        };

sub test {

print  "11111111111\n";

};

run(\&test);

[root@wx03 ~]# perl a2.pl

11111111111

11111111111

[root@wx03 ~]# perl a2.pl

11111111111

1: callback must be a CODE reference or another callable object at /usr/local/perl/lib/site_perl/5.22.1/x86_64-linux/AnyEvent/Impl/EV.pm line 50.

[root@wx03 ~]# cat a2.pl

use AE;

   use AnyEvent;

##定义watch

    sub run {

    my $code=shift;

    my $t = AnyEvent->timer(

        after    => 0,

        interval => 1,

        cb       => $code,

    );

my $cv = AnyEvent->condvar;

  $cv->recv;

        };

sub test {

print  "11111111111\n";

};

run(&test);

[root@wx03 ~]# perl a2.pl

11111111111

1: callback must be a CODE reference or another callable object at /usr/local/perl/lib/site_perl/5.22.1/x86_64-linux/AnyEvent/Impl/EV.pm line 50.