why does Subversion change file permission

Hi guys, I am working for a team that uses Subversion for version control,
and I meet an confusing issue. Here's the scenario:

I had a perl script named send_mail_daemon.pl, the file permission is 777
    -rwxrwxrwx 1 seth users 4662 Dec 6 14:23 send_mail_daemon.pl

then someone changed it and I update
> svn update send_mail_deamon.pl

after that, I found the file permission is CHANGED, which is not what I want
!
    -rwxr--r-- 1 seth users 4662 Dec 6 14:25 send_mail_daemon.pl

In fact, this changed file permission is an error in my project because some
one else cann't execute this script.

could someone please tell me why things go like this? and, any solution or
advice would be appreciate

thank you~~
Received on Thu Dec 6 07:57:25 2007