Nested loops become unwieldy when you have many arrays. With a suitable module it does not matter how many dimension the input has:

use 5.010;
use Set::Scalar qw();
my $iter = Set::Scalar->cartesian_product_iterator(
    map { Set::Scalar->new(@$_) }
        [1, 2, 3],
        [qw/a b c/],
        [qw/I II III/],
);
while (my @m = $iter->()) {
    say "@m";
}
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There are numerous alternatives: https://metacpan.org/search?q=cartesian+product

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Bonus:

use v6;
.say for cross (1, 2, 3), <a b c>, <I II III>
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