my %data;
$data{A} = [qw(a data goes here)];
$data{B} = [qw(b data here too)];
$data{C} = [qw(and here is c data)];

##</code><code>##

my %seen_in;
for my $key (sort keys %data) {
  for my $value (values %{$data{$key}}) {
    $seen_in{$value} .= $key;
  }
}