my %data;
my $header = <>;  # first line
while(<>) {
    my($key) = split /\t/;
    $data{$key} = $_;
}

# output:
print $header;
foreach my $key (sort keys %data) {
    print $data{$key};
}

##</code><code>##

perl thescript.pl file2.txt >file1.txt