Please do the following changes:-

foreach (@Geo_areas) {

      $_ or next;

      my $Return_results = "Select * from Result_storage_keep where Ag
+gregated_area LIKE '".$_."' \;";
                
      my $sth_C = $dbh->prepare($Return_results) or die "Couldn't prep
+are query: ".$dbh->errstr;
      
      $sth_C->execute() or die "Couldn't execute query: ".$sth_C->errs
+tr;
      
    while (my @row = $sth_C->fetchrow_array) {
    foreach my $val (@row) { 
      
      $val = '' unless defined;
    }
    print OUTPUT_FILE join("\t", @row);
    print OUTPUT_FILE "\n";
    undef @row;
      }
  }
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Don'y Use $_ for a list in the same loop for two times.