my %whole_file;
while (<ADDS>)
{
  my %info = (
    line_number => $.,
    line_text   => $_,
    atoms       => split /\t/
  );
  $info{key} = $info{atoms}[1];
  push @{$whole_file{$info{key}}}, \%info;

  # you can print out a tick every 100 or whatever here, using $.
}

my @dupes = grep { scalar @$_ > 1 } values %whole_file;

# to see what data structure I just built, dump it via your favourite dumper.  Mine is:
use Data::Dumper;
print Dumper(\@dupes);