#my $myhash;
#build_hash($myhash);
my $myhash = build_hash();
print Dumper($myhash);  ## shows %myhash to be empty

sub build_hash {
   my $href_ref;
   #my %results;
   
   my $sth = $dbh->prepare( $query );
   $sth->execute();

   # Get all records as array reference of hash references.
   my $res = $sth->fetchall_arrayref({});
   
   map { $hash->{$_+1} = $res->[$_] } (0 .. @$res-1); 

   #my $ct = 0;
   #while (my $tmp_href = $sth->fetchrow_hashref) {
   #   $ct++;
   #   $results{$ct} = $tmp_href;
   #}
   #%{ $href } = %results;
   ## also tried Storable's dclone(), but same results
   ## $href = dclone(\%results);

   print Dumper($href); ## prints desired results 
   return $href; 
}