return doesn't change @_.

@_ is an array of aliases to the actual arguments, so changing @_ changes the actual arguments.

sub { ++$_ for @_ }->(my @arr = 'a' .. 'c'); print "@arr";
# Output: b c d
[download]

In your case, it's bit more complex. Changing the function to

sub reverseArray {
    my @arr = @_;
    for my $i (0 .. $#arr) {
        @{ $arr[$i] } = reverse @{ $arr[$i] };
    }
}
[download]

wouldn't have helped, as you're working with an array of arrays. The inner arrays are represented as references, so even if you store the references in a different @arr, they still refer to the same inner arrays.

To keep the array unchanged, only assign to the copy, don't change the references from the original array.

sub reverseArray {
    my @arr = @_;
    for my $i (0 .. $#arr) {
        $arr[$i] = [ reverse @{ $arr[$i] } ];
    }
}
[download]

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