Variable look-behind-assertions were not available before 5.30. IMHO it makes no sense to combine a look-behind-assertion with \K.
\K comes handy in cases like 

echo fooooobar | perl -pe 's/fo{2,}ba\Kr/z/'

[download]

Greetings,
-jo

$gryYup$d0ylprbpriprrYpkJl2xyl~rzg??P~5lp2hyl0p$

In reply to Re: Practical use cases for \K in regex by jo37
in thread Practical use cases for \K in regex by perlboy_emeritus

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