# pass in _references_ to the arrays of lists @L and @R
# each element is the list of fields on a given line
COMPARE(\@L,\@R);

sub COMPARE {
    # $L is a reference to @L, $R is a reference to @R
    my( $L, $R ) = @_;
    # @Ret isn't used, it's just here to confuse you :)
    my @Ret = ();

    # %L is a hash with an entry for every index in @L
    # so we can skip lines already matched, and print
    # unmatched lines at the end
    my %L = map { $_ => $_; } 0..$#$L;
    # %R is a hash with an entry for every index in @R
    my %R = map { $_ => $_; } 0..$#$R;

    for my $I(0..$#$R ) {
        for my $J(0..$#$L ) {
            # if the @R line matches the @L line, based on
            # the key fields
            if($R->[$I]->[4] eq $L->[$J]->[1]) {
                # skip if we've already processed this @L
                # line
                next unless exists $L{$J};

                # delete these lines from %L and %H so we
                # don't print them at the end
                delete $L{$J};
                delete $R{$I};
                # print fields 5 and 6 of the R line, and
                # fields 2 and 3 of the L line
                print join ',', @{ $R->[$I] }[4,5], @{ $L->[$J] }[1,2];
                # go to next R line; this R line is already
                # matched
                last;
            }
        }
    }

    # print out the unmatched R lines, printing only fields
    # 5 and 6
    print join ',', @{ $R->[$_] }[4,5,0,0] for keys %R;
    # print out the unmatched R lines, printing only fields
    # 2 and 3
    print join ',', @{ $L->[$_] }[0,0,1,2] for keys %L;

}