Use substr:

#!/usr/bin/perl
use warnings;
use strict;
use feature 'say';

my $string = 'ABCDEFGH';
my $length = 4;
for my $start (0 .. length($string) - $length) {
    say substr($string, $start, $length), " -> Starting at position $s
+tart.";
}
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Note that Perl uses 0 for the starting position, not 1. If you really need 1, just output $start + 1.