my @list_A = (1..1000);
my @list_B = (1..1000);


# Exhibit A:

foreach my $element (@list_A, @list_B) {
    do_something($element);
}


# Exhibit B:

foreach my $element_A (@list_A) {
    foreach my $element_B(@list_B) {
        do_something($element_A, $element_B);
    }
}