First of all i would like to thanks for u guys givin me advises. Sorry if i dint make my problem clear there. :)
Actually beside Pg A, all is not my control anymore, they are code reside in other domain, so, what i need to do, is making a bot to send out a request (url), like
get_final_landing_pg(a_url)
Then, their system will based on my 'a_url' to do some processing, then they may have different redirect page based on the validity of my 'a_url',
let me make some explaination for a_url, actually it is a url with some id pass to their sys. Then their sys will determine what to perform based on my a_url's id, their sys will redirect to 'b_url' if my id got no problem, and other then 'b_url' will be 'not_b_url' shows 'Expired'
then, i can perform some comparison coz my sys already know the valid url 'b_url'(predefined in my system), coz i m still taking control in my a.pl.
Eg.
if (my_predefined_url eq get_final_landing_pg(a_url)) { return "not expired" } else { return "expired" }
There wont be any user interruption for the whole process like "click A" for redirect to pg_A or
"click B" for redirect to pg_B

Thanks for you guys spending time in reading my question. My broken english really brings up lots of trouble to you guys
:p