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;
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.
If you didn't program your executable by toggling in binary, it wasn't really programming!