$VAR1 = {
          'FOO' => {
                    'TABLE_NAME' => 'FOO',
                    'TABLE_TYPE' => 'REFERENCE'
                   },
          'BAR' => {
                    'TABLE_NAME' => 'BAR',
                    'TABLE_TYPE' => 'REFERENCE'
                   },
          'BAZ' => {
                    'TABLE_NAME' => 'BAZ',
                    'TABLE_TYPE' => 'CUSTOMER'
                   }
        };

##</code><code>##

$VAR1 = {
          'REFERENCE' => [
                           'FOO',
                           'BAR'
                         ],
          'CUSTOMER' => 'BAZ'
        };

##</code><code>##

my %HoA;
foreach my $key (keys %$ref_to_HoH){
   my $type = $ref_to_HoH->{$key}->{TABLE_TYPE};
   if (exists $HoA{$type}){
       push @{$HoA{$type}},$key;
   }else {
          $HoA{$type}  = $key;
   }
}

##</code><code>##

Can't use string ("FOO") as an ARRAY ref while "strict refs" in use at .....