Solution 5 (perl -nep 's/:CPONKT:(.*)/$1/;') is wrong.
With the same idea you could write :

perl -pe 's/.*:CPONKT:(.*)/$1/;' # or
perl -pe 's/.*?:CPONKT:(.*)/$1/;'# first one if ':CPONKT:'
                                 # is seen several times
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