You can turn your mapping on its head, by specifying it like this:

my %store_for_category = (
    "Dine Out" => ['restaurant A', 'restaurant B'],
    "Grocery"  => ['Walmart', 'HEB', 'Target'],
    ...
);
[download]

You can convert from the category to the store by:

my %category_for_store;
for my $category (keys %store_for_category) {
    for my $store (@{ $store_for_category{ $category }}) {
        warn "Duplicate category for store '$store': $category_for_sto
+re{ $store }, $category"
            if $category_for_store{ $store };
        $category_for_store{ $store } = $category;
    };
});
[download]

Usually, I end up wanting both mappings anyway, from "store" to "category" and to list all stores for a given category. I also tend to want to put a store into more than one category. I end up stuffing the pairs into an SQL database (like DBD::SQLite), but depending on where you keep the rest of the data, that may or may not be suitable for you.