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};
}
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perl thescript.pl file2.txt >file1.txt
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file1.txt output is the following:-
COMPUTER      DISTRIBUTION_ID    STATUS    
30F-WKS     `1781183799.xxx11'      IC---
30F-WKS     `1781183799.xxxx1'      IC---
ADM34A3F9   `1781183799.41455'      IC---
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I want 
COMPUTER      DISTRIBUTION_ID    STATUS    
30F-WKS     `1781183799.xxx11'      IC---
ADM34A3F9   `1781183799.41455'      IC---
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Like someone said in the Chatterbox: your data may not separated by tabs. Therefore, the whole record (line) would be treated as the id.

Replace split /\t/ in my code, with split /\s+/.

If it still won't work, then use the following code at the end, to test what's in the hash:

use Data::Dumper;
print Dumper \%data;
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and see what makes it fail.

If you need unique across an entire set, no question that hashes are most useful. Problem, though, is that you have to then store all the keys.

It is not uncommon to want to dedup when there are successive runs (think unix's 'uniq'). That's when this second class comes into play. Set a state variable, and read one line at a time. You may have to keep around the previous line or two to compute your state. You may have to do some work at the end to clean up stored lines.

my $thisKey;
my $lastLine = <>;
my $lastKey = '';  # first line is header, so always print
while (<>) {
    if (/(.*?)\t.*/) {
        $thisKey = $1
    } else {
        warn "bad data: $_ had no tab\n";
    }
    if ($thisKey ne $lastKey) {
        print $lastLine;
    }
    $lastLine = $_;
    $lastKey = $thisKey;
}
print $lastLine;
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