The status check can be done using e.g. Net::Ping. How to do the redirection depends on what webserver you use and whether you want to redirect just one page or a whole bunch of pages.

Update: Here's a quick&dirty script for the first case:

use Net::Ping;

my $host = "www.server.com";
my $alive_url = "http://www.server.com/somepage";
my $dead_url = "http://www.backupserver.org";

my $p = Net::Ping->new();
my $url = $p->ping($host) ?
    $alive_url : $dead_url;
$p->close();

print <<EOT;
Status: 302 Moved Temporarily
Content-Type: text/html
Location: $url

<p>Go to <a href="$url">$url</a>.</p>
EOT
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