If you want to have the delimiter as a separate list item, then use capturing parenthesis around it.

If you want to have it attached to the previous item, you need to make it a zero-width look-behind:

split(/(?<=[KR])(?!P)/, $a);
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Also you have a few errors in your script that prevent the snippet you posted from producing the output you show. (Mis-named variables, missing semicolon).