Using split to cut up the string but capturing the delimiters so they do not get lost.

use strict;
use warnings;

my $str="AJKDHAKAESRADADKLASRRASDASDKASEKA";

#split on K or R and keep the delimiters
my @parts = split /([KR])/, $str;
print "@parts\n\n";

# return parts in groups so each group has 2 K or R
my $start = 0;
foreach my $end (1 .. $#parts / 4) {
    print join '', @parts[$start .. ($end * 4 - 1)], "\n";
    $start = $end * 4;
}

print "\n";

# return parts in groups so each group has 3 K or R
$start = 0;
foreach my $end (1 .. $#parts / 6) {
    print join '', @parts[$start .. ($end * 6 - 1)], "\n";
    $start = $end * 6;
}
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Can you see the pattern emerging?

Or using a single regex:

use strict;
use warnings;

my $str="AJKDHAKAESRADADKLASRRASDASDKASEKA";

my @parts = $str =~ m/((?:[^KR]*[KR]){2})/g;
print "@parts\n\n";

@parts = $str =~ m/((?:[^KR]*[KR]){3})/g;
print "@parts\n";
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