use List::Util qw(max min);

my %id_hash;
open (DATA, ".txt"); 
while (<DATA>) {
    chomp;
    my ($order, $mtime, $size, $id, $date) = split /\t/;
    push @{ $id_hash{$order}{$id}{mtime} }, $mtime;
    push @{ $id_hash{$order}{$id}{size  } }, $size;
    push @{ $id_hash{$order}{$id}{date   } }, $date;
}

open (OUT, ">output.txt"); 
for my $order (keys %id_hash) {
    for my $id (keys %{ $id_hash{$order} }) {
        my $Low     = min( @ { $id_hash{$order}{$id}{mtime} } );
        print OUT "$order $Low \n";
    }
}