use v5.12;

my $check = "a";
$check++ for 1..3;  # three letters
say $check;

$check++ for 1..24;  # another 24 letters, but there's only 26 in the alphabet,
say $check;          # so what happens now we've run off the end?

# hmmm... that's handy.
# it takes us to the next part of your question...

my $input = $check;
my $output = substr($input, 1, 1); # skip one character, take one character

say $output;