my %hash = qw(1 a 2 b 3 c);
my $fixed_href = {};

fix_hash(\%fix_hash, $fixed_href);

sub fix_hash {
  my ($old, $new) = @_;
  while (my ($key, $value) = each %$old) {
    (my $nkey = $key) =~ s/^(?=\d)/N/;
     if (ref($value) eq 'HASH') {
       $new_value = {};
       $new->{$nkey} = $new_value;
       fix_hash($value, $new_value;
     } else {
       $new->{$nkey} = $value;
     }
  }
}