while (1){

   $input = <STDIN>;

   print $input; 

   #This is where I would check for CTRL+D
   #last if ($input equals to CTRL+D); EXIT LOOP

   if($input > 0){
    print " is positive\n";
   }

   elsif($input < 0){
    print " is negative\n";
   }

   else { print " is zero\n"; }
}

解决方法

while (defined($input = <STDIN>)) {
    ...
}

while (defined($input = <>)) {
    ...
}