FWIW, this is what I get by wrapping each “result” in a sub noop {}; like, noop("$i$j$k$l") and noop(sprintf "%04d", $_) to take out any IO influence.

          Rate    unpack     split     nested    sprintf
unpack    102/s       --       -1%       -80%       -84%
split     103/s       1%        --       -80%       -83%
nested    513/s     401%      397%         --       -18%
sprintf   625/s     510%      506%        22%         --
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