Sign Clean Messages not working without -U

[Ram]

I have  MailScanner version 4.84.5 on CentOS 6

Even if I set in MailScanner.conf

Sign Clean Messages = yes

messages  dont get signed.
I enabled the -U switch in /usr/sbin/MailScanner and now it works but 
with warnings
like this

Insecure dependency in open while running with -T switch at 
/usr/lib64/perl5/IO/File.pm line 185.


What does this mean ?  Is -U not recommended ?



Thanks
Ram