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

$/ = '/Hit_accession>';

while(<DATA>){
  if( m{<Hit_def>(.+?) odorant-bin} ){
    my $match = $1;
    s{(<Hit_id>).+?(</Hit_id>)}{$1$match$2};
  }
  print $_;
}

__DATA__
<Hit_num>6</Hit_num>
<Hit_id>gnl|BL_ORD_ID|3665984</Hit_id>
<Hit_def>gi|158703516|gb|ABW77886.1| odorant-bin...
<Hit_accession>3665984</Hit_accession>
<Hit_num>7</Hit_num>
<Hit_id>gnl|BL_ORE_ID|3665984</Hit_id>
<Hit_def>gi|153326716|gb|ABF88997.2| odorant-bin...
<Hit_accession>3665984</Hit_accession>