use strict;
use warnings;

my $line_count = 10;

my $line = join ("", 'a'..'z',"A".."Z") . "\n";

my $file= "$line" x $line_count;



my $offset = 2;
my $rest   = length($line) - $offset -1;


my $fmt = qq{(x${offset}ax${rest})${line_count}};

my @results = unpack $fmt, $file;

print @results;