In scalar context, glob iterates through such filename expansions, returning undef when the list is exhausted.

for (1..4) {
   if (glob($0)) {
      print("I exist\n");
   } else {
      print("I don't exist\n");
   }
}
[download]

outputs

I exist
I don't exist
I exist
I don't exist
[download]

if (glob(...)) is wrong. The only time glob should be in scalar context is in while (glob(...)), and the loop better not execute last, return or die.

Fix:

for (1..4) {
   if (() = glob($0)) {
      print("I exist\n");
   } else {
      print("I don't exist\n");
   }
}
[download]

or

for (1..4) {
   if (-e $0) {
      print("I exist\n");
   } else {
      print("I don't exist\n");
   }
}
[download]

I exist
I exist
I exist
I exist
[download]