Perl has a 'magic' incrementor. The code you posted for letters will work if you change the comparison operator <= to its string equivalent le:

use strict;
my $start_letter = 'A';
my $end_letter = 'D';
for (my $i=$start_letter; $i le $end_letter; $i++) {
  print "\$i = $i\n";
}
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As others have stated there are better ways to do this in Perl...