Note that Perl 6 flattens less eagerly, and tries hard to preserve information:

$ cat flatten.pl
sub mysub(@a1, @a2) {
    say "a1: ", @a1.join(', ');
    say "a2: ", @a2.join(', ');
}

mysub((1..5), (6..9));

$ ./perl6 flatten.pl
a1: 1, 2, 3, 4, 5
a2: 6, 7, 8, 9
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