Of course, if the question is being asked in the context of reading STDIN from a pipe (rather than from a keyboard), then there could be a good reason to figure out whether one method of reading is faster or slower than another, and a benchmark test would be worthwhile -- even if all it does is prove that there's hardly any difference.

And in that regard, the Benchmark module probably isn't necessary or even appropriate; the unix "time" command would probably do. Just put together a suitable test script that reads and processes data from STDIN, but accepts a command-line option to determine what sort of syntax to use for reading, then run a series of commands like:

feeder_process | time test-perl-reader diamond > /dev/null
# (repeat several times, average the results)

feeder_process | time test-perl-reader readline1 > /dev/null
# (repeat, average the results)

feeder_process | time test-perl-reader readline2 > /dev/null
# (you know the drill...)
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If the difference among the various averages is greater than the variance among test runs for any single method, then maybe there's a real difference in the efficiency of the different input methods.

But I would expect any differences to be a very small fraction of the overall pipeline time.