use Data::Dumper;
my %hash;

my $client = "client1";
$hash{$client}{fieldA} = 12234;

#from another query:
$hash{$client}{fieldC} = 9876;

print Dumper \%hash;
__END__
$VAR1 = {
          'client1' => {
                         'fieldA' => 12234,
                         'fieldC' => 9876
                       }
        };

##</code><code>##

use Data::Dumper;
my %hash;

my $client = "client1";
$hash{$client}{fieldA} = 12234;

#from another query:
$hash{$client}{fieldC} = 9876;

my @order = qw (fieldC fieldB fieldA);

foreach my $client (keys %hash)
{
   print "$client";
   foreach my $field (@order)
   {
      if (exists $hash{$client}{$field})
      {
          print "\t$hash{$client}{$field}";
      }
      else
      {
         print "\tno_value";
      }  
   }
   print "\n";
}
#prints: client1 9876    no_value    12234