[NOT SPAM] how spamassassin evaluates "eval"

[jaysheel bhavsar]

Hi,
  I wanted to know how does spamassassin evaluate "eval" from the .cf files. 
when spamassassin see something like the following 
----------------------------------------
header FAKE_HELO_AOL            eval:check_for_rdns_helo_mismatch("aol
\.com","aol\.com")
-------------------------------------------

I tried to read the parser file but I could not understand it as I am not good 
at perl.

Thanks
Jay.