To summarize revdiablo
's results: if it's going to take more than 1/30000th of a second, File::Slurp is faster; otherwise the idiomatic slurp may be faster. Sounds like the OP here caught a serious case of premature optimization (ironically, while somewhat arguing against it).
There are other ways to measure "efficiency". For instance, I know the difference between $/ and $\, but I'm still prone to mistype one for the other perhaps as much as 1% of the time. The time it takes to identify and correct this is a huge inefficency; just using File::Slurp eliminates this problem, at least in the case of file slurping.