...
my @s = split /
               (?<=[KR])  # split on K,R
               (?!P)      # but not if in front of a P
              /x, 'APADPKGSTIDRPDAARTLTVHKCEQTDTRGVKEGTRNEDPQAECKPVSDVEFTITKLNVD';

print join "\n", @s;
...