my $a = "AAAAXRXAAAAAXKXAAAAAXRPXAAA"  # sample sequence
@digested0 = split(/[KR](?!P)/, $orf); # cut at every K or R, except if followed by a P
print join("\n",@digested0);

##</code><code>##

AAAAX
XAAAAAX
XAAAAAXRPXAAA

##</code><code>##

AAAAXR
XAAAAAXK
XAAAAAXRPXAAA