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while (my $r = $attr_detail->fetchrow_hashref()) {
  $details->{$r->{'masterid'}}->{$r->{pco_attribute_id'}}->{pco_value}
+ = $r->{'pco_value'};
...
    print DETAILS "$master_id\t$attr_id\t$pco_value\n";
  }
}

for my $m_id_attr_id (keys %details) {
  my ($m_id, $attr_id) = split $;, $m_id_attr_id;
  print DETAILS "$m_id\t$attr_id\t$details{$m_id_attr_id}\n";
}