#! perl -slw
use strict;

my $re = '([^X]+)X*' . '(?:([^X]+)X*)?' x 100;
$re = qr[$re];

my $screen = "ATCGATCGXXXXXATCGATXXXACTGCTACGGTACXXXAATTATXGCGCGXXT";

$screen =~ $re;

print for @-[ 1 .. $#- ];

__END__
P:\test>test2
0
13
22
38
45
52