Its not entirely clear for me what you try to do.

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

print join "\n", @s;
...
[download]

The above would be my first guess here.

Regards

mwa