A solution: mind you, I wouldn't use this on a very big file. Not exactly the method davido was describing, but it will do.

my %id_hash;
my @lines = ();
open (DATA, "test.txt");
while ($line = <DATA>) {
    chomp $line;
    my @line = split /\t/ , $line;
    push @lines, \@line; # push the original line in an array as an an
+onymous array
    if ($line[1] > $id_hash{$line[0]}) {$id_hash{$line[0]} = $line[1];
+} # calculate the biggest mtime for a given order id
}

open (OUT, ">output.txt"); 
foreach my $item (@lines) {

    my @line = @{$item};    # get the original line
    $line[4] = $id_hash{$line[0]}; # replace the fifth element with th
+e calculated maximum
        print OUT join "\t" ,@line,  "\n"; # print the adapted line
}
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Greetings

Martell