Very basic and better answered for yourself by reading perldoc perlintro (from your command line)... and -- as is often the case -- correctly answered in many different ways. Here is a (partial) answer for each question:

my $check = "A";
for (1..2) {
    $check++;
}
say $check;

#2:

my $input = "xy";
$input = reverse ($input);
chop $input;
say $input;
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The first, obviously, is a simplied loop using the range operator (alternate syntax is shown by other replies above); the second takes advantage of the otherwise little-used chop. Note that it will "work" even if your charpair is NOT an ordered pair.

Please take these for examples, and set yourself the task of finding at least two other ways to answer each of your questions.