One trick is to normalize the date so you can compare it directly:

use warnings;
use strict;

my %bestLine;

while (<DATA>) {
  my ($line, $month, $day, $year) = /(\d+)\s+(\d{2})(\d{2})(\d{4})/;
  my $date = "$year$month$day";
  
  $bestLine{$line} = $date if ! exists $bestLine{$line} or $bestLine{$
+line} < $date;
}

print join "\n", map {"$_: $bestLine{$_}"} sort keys %bestLine;

__DATA__
13               10102005
13               09152005
13               11052005
200              11012005
200              11152004
[download]

Prints:

13: 20051105
200: 20051101
[download]

Update: revise code to match OP's problem.