G'day truthseeker66,

++toolic's answers are spot on. Here's some additional information.

"When do I use 'my' before an array and when don't I?"

While you're focussing on arrays with this specific question, the information provided applies equally to declaring other types of variables, e.g. my $scalar, my @array and my %hash.

So, as already stated, you declare your variable once then subsequently use it as many times as you want (without further declaration). If you declare a variable more than once you'll usually get a warning like this:

$ perl -Mstrict -Mwarnings -e '
my $x;
my $x;
'
"my" variable $x masks earlier declaration in same scope at -e line 3.
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[Advanced usage: there are ways to declare variables with the same name more than once within the same piece of code - that's rarely, if ever, needed and best avoided as it's confusing and will often lead to unexpected errors.]

my is the most frequently used way to declare variables - more in-depth information can be found in perlsub (particularly under Private Variables via my()).

perlsub also has a lot of information about other ways to declare variables, including local, our, constant and state.

"Does this code '{$a <=> $b}' work with only numbers?"

You can reverse the order of the sort by swapping $a and $b:

$ perl -Mstrict -Mwarnings -E '
my @numbers = (2, 1, 3);
my @strings = qw{B A C};
say sort { $a <=> $b } @numbers;
say sort { $b <=> $a } @numbers;
say sort { $a cmp $b } @strings;
say sort { $b cmp $a } @strings;
'
123
321
ABC
CBA
[download]