Lagging for loops

input

Sandy,grocery store,tomatoes
Sandy,grocery store,cheese
Sandy,train station,tickets
Ben,train station,tickets
Ben,video game store,portal II

lagging-for.pl

given open('input') {
    my @shop-list;
    my ($last-seen-person, $last-seen-shop);
    for .lines -> $line {
        my ($person, $shop, $item) = $line.split(',');
        if $person ne $last-seen-person || $shop ne $last-seen-shop {
            if defined $last-seen-shop {
                push @shop-list, $last-seen-shop;
            }
            $last-seen-shop = $shop;
        }
        if $person ne $last-seen-person {
            if defined $last-seen-person {
                say ($last-seen-person => @shop-list).perl;
            }
            @shop-list = ();
            $last-seen-person = $person;
        }
    }
    if defined $last-seen-shop {
        push @shop-list, $last-seen-shop;
    }
    if defined $last-seen-person {
        say ($last-seen-person => @shop-list).perl;
    }
    .close;
}

# Problems with the above code:
#
# A. Two variables whose only purpose is to keep enough lagging state.
# B. Tricky conditionals involving these variables.
# C. Duplicated code outside of the loop.
# D. The `@shop-list` variable needs to have a scope outside of the loop.
# E. Lots of accidental complexity due to A-D.
# F. Easy to get things wrong due to E.

# What I'd like to be able to write:
#
# given open('input') {
#     for .lines -> $line {
#         my ($person, $shop, $item) = $line.split(',');
#         OVER $person {
#             my @shop-list;
#             OVER $shop {
#                 push @shop-list, $shop;
#             }
#             say ($person, @shop-list).perl;
#         }
#     }
#     .close;
# }