You know, that gives me another idea for the cases you need to conserve the last field, although it is so similar to the add a bogus field approach that it bears the question whether the capture/grep hoopla is worth the hassle if you went that route anyway: add a field separator to the string.

$ perl -le'$_ = "2|3|||||"; print join " : ", grep ++$i%2, split /(\|)
+/, "$_|"'
2 : 3 :  :  :  :  :
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Makeshifts last the longest.