Since it's not https-related, you're probably back to wanting a fatalsToFile equivalent
It turns out that it is https-related...
There were two red herrings.
Firstly the .htaccess uses a 301 redirect that was being cached and also the getstore method of LWP::Simple is not doing what I expected.
my $availability;
my $response = LWP::Simple::getstore("http://$ENV{'HTTP_HOST'}/cgi-bin
+/booking.pl?command=check&st=$data{'st'}&ed=$data{'ed'}&force=$data{'
+force'}", $availability);
print "->$availability<-\n";
print "$response\n";
Shows
$availability as empty even though the server is responding with 200 code.
So, now I know that it is an HTTPS issue, I need to find a way to fix it...