use strict;
use Data::Dumper;

my $string="abcdefghijklmnopqrstuvwxyz";
my $aryref= [];
chunk_em($string,$aryref,5);
print Dumper($aryref);

sub chunk_em {
  
  my $string=shift;
  my $aryref=shift;
  my $num=shift;
  my ($first, $second)=unpack("A${num}A*", $string);
  push @$aryref, $first;
  chunk_em($second,$aryref,5) if $second;
  return $aryref;
}
__END__
$VAR1 = [
          'abcde',
          'fghij',
          'klmno',
          'pqrst',
          'uvwxy',
          'z'
        ];