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

Only when you first declare the array variable.

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

Yes. You want cmp (perlop and sort)

use warnings;
use strict;

my@array = qw(one two three four five six seven eight nine ten);
print "The original array contains - @array \n";
my@sorted = sort {$a cmp $b} @array;

print "The sorted array contains @sorted \n";
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You should also use "code" tags for your error/warning messages.