#!perl

use strict;
use warnings;

my $string = qq[FGTXYZGTFABCGHABCFGTXYZADXYZGTYABC];

while ($string =~ /[XYZ]{3}([A-Za-z]{0,21}?)[ABC]{3}/g) {
   print $-[0] . ', ' . length($1) . $/;
}
__OUTPUT__
3, 3
20, 8