The program should die properly if failed to open (unless the pipe fools the call into thinking it went through ok).. but permissions are definitely a factor. Most servers run as "nobody", so you need to make sure that group and other have the permissions you need.
also, for troubleshooting, you can try:

while(<TMP>) {
    ($receiver,$uptime) = split;
    if ($DEBUG == 'TRUE') {
      # test split values, make sure you've 
      #  printed a header already
      print "testing: reciever = $reciever : uptime = $uptime<BR>";
    }
    push @dataset, $uptime;
    push @labels, $receiver;
}
[download]

that way you can see what it's doing as it's running..
remember to print out the vars as it's working if you're not getting what you think you should be..
-Syn0

Update: Thinking about the whole user issue, it may be that server user (probably nobody) may not be have access to obtain the information from uptime.pl (it may be able to run the script, but not get the data..)
just another thought..