Untested, but something like this should be well enough:
in reply to Help needed on HTTP redirects
head should return true (and some info) only if the site asked is up and working. If it isn't enough, you might want to look at LWP head replacement, especially merlyn's answer.
use CGI 'redirect';
use LWP::Simple 'head';
print redirect('/404'); # already at siteA
You have moved into a dark place.
It is pitch black. You are likely to be eaten by a grue.