my @runs;
my $pos = 0;
my $b = $a;
my $c = lc substr $b, 0, 1, '';
my $next;
while ( $b ) {
    my $start = $pos;
    1 while ++$pos and ( $next = lc substr $b, 0, 1, '' ) eq $c;
    for my $i ( $start .. $pos - $n ) {
        push @runs, substr $a, $i, $n;
    }
    $c = $next;
}