my %id_hash;
my @lines = ();
open (DATA, "test.txt");
while ($line = <DATA>) {
    chomp $line;
    my @line = split /\t/ , $line;
    push @lines, \@line; # push the original line in an array as an anonymous array
    if ($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 the calculated maximum
        print OUT join "\t" ,@line,  "\n"; # print the adapted line
}