You do not have 2 array parameters, you have 2 lists. And the answer is: because Perl is designed to flatten lists - and arrays in list contexts are lists with the array elements. The answer to the second question is: you cannot. You can, however, pass two references to arrays to a sub:

mysub([1..5],[6..9]);
sub mysub {
    my @array1 = @{$_[0]};
    my @array2 = @{$_[1]};
}
[download]