If all files in the directory are newer than the beginning of the script execution, you'll get no results. That's why you shouldn't initialize your "oldest" to 0, but rather to the first element of the array, if any:

my @names = glob "*" or die "Nothing to scan";
my $oldest_name = shift @names;
my $oldest_age = -M $oldest_name;
for (@names) { # remaining names
  if (-M > $oldest_age) {
    $oldest_name = $_;
    $oldest_age = -M _;
  }
}
printf "%s is %.2d days old\n", $oldest_name, $oldest_age;
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Why does this look familar? Why, it's one of our Llama exercises!