my $string = "a(b)cd(e)f"; 
my @parts = split /(?=.{5}$)/, $string';

##</code><code>##

my $string = "a(b)cd(e)fg(h)ij(k)l"; 
my @parts = split /(?=(?:.{5})+$)/, $string';

##</code><code>##

my $string = "a(b)cd(e)fg(h)ij(k)l"; 
my @parts = $string =~ /(.{5})/g;