You have to stop using & in your exec, since that creates yet another pid.

exec ('/usr/bin/zenity', '--display=:0.0', '--info', '--title=Reminder
+', '--text=' . $reminder) or die ...
[download]

I suspect that your $pid++ is assuming that nothing else gets launched between your fork/exec and the shell's parsing and fork/exec of zenity.

That all said: formatting! And try Proc::Fork or something to simplify your child/parent handling.