my @a1 = qw/a b c d e f g/;
my @a2 = qw/h i j k l m n/;


sub mysub { 
    my ($r1, $r2) = @_;
    
    # make a copy of the first array
    my @a1_copy = @{ $r1 };
    $a1_copy[2] = 'blah';   # only affects this copy

    # tweak the original @a2
    $r2->[2] = 'blah;   # affects @a2

}

mysub(\@a1, \@a2);