The repetition of the test makes it highly probable that the file is cached in RAM. On one side, this means that the results you get are probably indicative of the real performance of the software portion of the reading process, i.e. the comparison is meaningful to some extent. OTOH, real-world situations in which you have to read gigabytes of data will probably suffer a bottleneck from the actual reading from the device, so the differences would probably disappear.

The interesting thing is in your answer to the node you pointed. You do this because that's how you learned how to do it - because that's how you see how to do it almost everywhere (apart from the use of File::Slurp). This is no cargo cult IMHO, but a useful Perl-level idiom to say "slurp that file"; to this extent, using read would be way too "low-level"!