use strict;
use Data::Dumper;
use Carp;

my %hold;

while ( <DATA> ) {
    chomp;
    next if (/^\#/);
    my @elem = split(/\t/,$_);

    # Keep in Hash of Array
    # HoA here consumes so much memory
    push @{$hold{$elem[0]}}, $elem[1];

}               

# is there a better way to keep/process the array
# other than HoA like %hold ?


foreach my $key ( sort keys %hold  ) {

    my @ary = @{$hold{$key}};

    # then I will process @ary for each key above
    print "$key\n";

}               


# in practice there are ~4-5Gb of such lines below

__DATA__
#Tags								Error_Rate_In_ASCII
AATACGGCCACCCCCCCCCCCCCCGCCCCTCCCC	INILILFIIIIQNQQNQNLLKFKNCDHA?DAHHH
CTTTCCCTCCACGACGCTCTTCCGCTCTCATGAT	QQIQQQQQIQQQIQQLQNQNOPNKIHHHAHHAAA
TCCACTCTTTCCCTACACGACGCTCTTCCGATCT	QFQFQQQQQQQQQQQQIQLFNNPONHHHHHDHHH
TCCCCTCTTTCCCTACACGACGCTCTTCCGATCT	UIUIUUUUUUUUUULUUUIOUKUNULLLLKKLLK
TGATACGGCGACCACCGAGATCATCACACTTTCC	UUUUUUUUUUUUUQUUTUUUUULLUKRHPKIHHO
TGATACGGCGACCACCGAGATCTACACTCTTTCC	UOIUIUUUUUUUUIUUUOUOUUUUUKLLLLIKKL
TGATACGGCGACCACCGAGATCTACACTCTTTCC	UUUUMUUUUUUUUIUUIUUQUUUUUOOOOOOOOO
TGATACGGCGACCACCGAGATCTACACTCTTTCC	UUUUUUUUUUUUUUUUUTUUUUUUURRRRRMPPQ
TGATACGGCGACCACCGAGATCTACACTCTTTCC	UUUUUUUUUUUUUUUUUUUUUUTUURRPRRIMQQ
TGATACGGCGACCACCGAGCTCTACACTCTTTCC	UUQUUUUUUUMUUUUUUQUUUUUUUOOOOOIOOO
AATTCTGCGCCCCCCCCACTCAGCCCCCCTCCCC	LFNFQNQNFLQLIQQLIIQNOCIIIAAAAAHHHA
AGATACGGCCACCACCGAGATCTACACTCTCTCC	NFQNIQLFQIFNQNQQFQQNNKKINAHAHH?AHD
TGATACGGCGACCACCGCGATCTACACTCTCTCC	UUUUUUUUUUUUUUUUTLUUQUUCUPRRRRHRNQ
TGATCCGGCGACCCCCGAGCTCTACACTCTTTCC	QQQQIQQIQQQQQNQQQQQLOOKNPHHHHHHDHH
TGCTCCGGCGACCACCGAGATCTACACTCTTTCC	QQIQFQQNQQQQQIQQQLQLNOKIOHHHHHADHH
TGCTCCGGCGACCACCGAGATCTACACTCTTTCC	UIOUOULOUUUUUOUUUOUUUUUUULLKLLIGLL
TGCTCCGGCGACCACCGAGATCTACACTCTTTCC	ULOUIUOUUUUUUUUUULUUUUUUULLLLLIGLL
GTCTCCTGCGACCCCCGAGCTCTACACTCTTTCC	QLLQIQIFQNQQQIQQNQNLOONNOHHHHHHHHH
TTCTCCTTCGACCACCGAGATCTACACTCTTTCC	QLNQIQLIQINQQQQQQLQQOPONOHHHHHHHHH
TTCTCCTTCGACCACCGAGATCTACACTCTTTCC	UOUUIUOIUILUUUUUULUUUUUUULLLLLKLLL