my %lines;

read_results("file1.txt");
read_results("file2.txt");

print "$lines{$_}\n" for sort {$a <=> $b} keys %lines;

sub read_results {
    my $fn = shift;

    open my $FH, "<", $fn or die "couldn't open $fn: $!";

    while (my $line = <$FH>) {
        chomp $line;
        my ($id) = $line =~ /TAT_(\d+)/;
        next unless $id;
        $lines{$id} = $line;
    }
}