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use v5.12;
use warnings;
...
  $str =~ /( (?: (.) (?{COUNT}) )* (*FAIL) )/x;
  say "$l : $c = ", 2 *$l-1;
}

# perl /home/lanx/perl/pm/recursive_regex.pl
abcdefghijklmnopqrstuvwxyz
...
24 : 300 = 300
24 : 47 = 47