However 'Ignoring caches' and and other real world aspects of programming leads to bad decisions. In this case for example caching means that the line at a time technique is likely to scale very well for extremely large files whereas slurping the files is likely to lead to thrashing the hard disk even for fairly modest (by today's standards) file sizes.

In general slurping is a bad design choice and for a line by line task such as indicated by the OP Tie::File is likely to be (at best) little more than syntactic sugar, and at worse may impose significantly more overhead than the multiple file handle solutions already offered.