That

  my @C = map { [ $_ ] } @{ shift @_ };
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Can be simplified to:

  my @C = [];
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Don't believe me? Try it. Also, it now works correctly in the case you pass no arguments to the function. This may be a matter of taste, but I prefer the leftmost array to be the outermost loop. So that leaves us with:

sub cartesian {
    my @C = [];

    foreach (reverse @_) {
        my @A = @$_;

        @C = map { my $n = $_; map { [ $n, @$_ ] } @C } @A;
    }

    return @C;
}
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