my @order = ();
my %hash = ();
while (<DATA>) {
    # split line on tab
    my @data = split /\t/;
    
    # for keeping the order
    push @order, $data[0] unless $hash{$data[0]};

    # make an array ref if there isnt' one
    $hash{$data[0]} = [] unless $hash{$data[0]};

    # store it away for later
    push @{$hash{$data[0]}}, $data[1];
}

# keeping the order here
for my $col1 (@order) {
    # get that stored array
    my $aref = $hash{$col1};
    
    # print tab-delimited, grouped on col1 and in order
    print join("\t", $col1, @$aref), "\n"';
}