First you need to be 'security aware*' which seems to be the case for you.
Then use the community experience
(Super Search will teach you to : use strict, learn taint checking, not store plaintext password...)
or security sites (SecurityFocus for example) archives/mailing lists

Then perldoc perlsec is a must read...

Eventually, I suppose peer review is the best way...
(How do you call it ? Open source ? ;-)
The monastery, CPAN or any scripts archive site is ok for such review as long
as you label your work as alpha and ask for feedback

* I put this in Bold for all the french Jean-claude Vandame's fan ;-)

"Only Bad Coders Code Badly In Perl" (OBC2BIP)