Timeout
        Description:
            If no new requests have been received after <number>
            seconds, exit the persistent perl interpreter.  Zero
            indicates no timeout.

なので，こっちのような気がします．

my $maxspan = 600 ;
my $span = $GBX->{NOWTIME} - $FOX->{NOWTIME};
if($span > $maxspan) {$sp->shutdown_next_time;}