#!/perl -l
use strict;
use warnings;

my %merge;

while ( <DATA> )
{
    chomp;
    my ($key, $value) = split /\t/;
    $merge{$key}{$value} = $value;
}

for my $key ( keys %merge )
{
    my @values = keys %{$merge{$key}};
    print "$key => @values";
}

__DATA__
1   101
2   102
3   103
1   104
2   102
3   104


__output__
1 => 104 101
3 => 104 103
2 => 102
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Update: First draft was wrong, misread of the question. Updated within 2 minutes. Apologies if anyone read the first version. :)