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

##</code><code>##

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_store{ $store }, $category"
            if $category_for_store{ $store };
        $category_for_store{ $store } = $category;
    };
});