Is there a way for a box behind a firewall to tell what IP address the firewall is using

As DamnDirtyApe notes, a static hostname is a good way of solving this particular problem, but an alternative is something like...

use LWP::Simple;
if (get('http://www.whatismyip.com/') =~ /(\d+[.]\d+[.]\d+[.]\d+)/) {
    print $1;
}
[download]

Another solution (if you have control over the firewall), is to install SNMP and then query the firewall remotely using Net::SNMP (alt.) or Net::SNMP::Interfaces (alt.).

    --k.

Update: Fixed snippet.