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
                       }
        };