Untested, but something like this should be well enough:
use CGI 'redirect';
use LWP::Simple 'head';
print redirect('/404'); # already at siteA
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
You have moved into a dark place.
It is pitch black. You are likely to be eaten by a grue.