assuming your script goes into some sort of work loop you can check how long you've been running and exit if needed.

...
my $stop_time = time + 60 * 60 * 7; # now + 7 hours
while (1) {
    do_work;
    last if time > $stop_time;
}
...
[download]

your script will stop the first chance it gets.

if you need immediate gratification the script will need to write a pidfile so you'll know which script to kill. (if there's only ever a single instance of your script running you can get away with the 'killall' solution.)

another alternative would be something like this...

...
while (1) {
  do_work;
  if (-f '/tmp/script.die.die.die') {
    unlink '/tmp/script.die.die.die';
    last;
}
...
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then you could have a crontab entry at 4p.m that just does a 'touch /tmp/script.die.die.die' and again your script would stop as soon as possible.