#!/usr/bin/perl 
use strict;
use warnings;

use Bio::DB::EUtilities;
use Bio::SeqIO;
use Bio::Seq::SeqBuilder;

my $file3 = 'na_gi_numbers.txt';

my $in = Bio::SeqIO->new(-file => $file3, -format => "genbank");
   my $builder = $in->sequence_builder();
   $builder->want_none();
   $builder->add_wanted_slot('accession_number');
   for (1..1000) {
      my $seq = $in->next_seq;
      print $seq->accession_number."\n";;
   }