Perl exec(‘/usr/bin/php -v’)挂在CentOS 6.6上,除非STDIN先关

环境：

# cat /etc/*-release
CentOS release 6.6 (Final)
...
# perl -v 

This is perl,v5.10.1 (*) built for x86_64-linux-thread-multi
...
# php -v
PHP 5.4.41 (cli) (built: May 14 2015 23:15:28) 
...

mini.pl：

#!/usr/bin/perl
exec('/usr/bin/php -v');

运行它并查看挂起的PHP进程：

# perl mini.pl &
[2] 16958
[1]   Killed                  perl mini.pl
# ps ax | grep 16958
16958 pts/2    T      0:00 /usr/bin/php -v
16960 pts/2    S+     0:00 grep 16958

[2]+  Stopped                 perl mini.pl

但是如果我关闭STDIN：

mini.pl：

#!/usr/bin/perl
close(STDIN);
exec('/usr/bin/php -v');

运行正常

# perl mini.pl &
[1] 16976
# PHP 5.4.41 (cli) (built: May 14 2015 23:15:28) 
...

[1]+  Done                    perl mini.pl
# ps ax | grep 16976
16978 pts/2    S+     0:00 grep 16976

其他注意事项

>我无法在Mac OS X上重现这种挂起的行为(perl 5.18.2,php 5.5.24)

谢谢,