virtualsue is right about why slurping large files is slow, but the above code will be slower for good reason.

As a general rule, the more detailed the instructions that Perl gets, the slower it will be. The reason is that Perl is interpreted, and so it is constantly going back to your instructions, figuring out what to do next, and then doing that. But the more that Perl is getting instructions that allow it to "chunk" operations, the easier it is for Perl to do that efficiently.

Think of yourself as perl and this becomes obvious. In the one case you are told to grab a hunk of data in lines, allocate an array, and shove the data there. In the other case you are told to open a file, scan in a line, alias that to $_, append to an array (do we need to allocate more for the array now?) etc.

Which instructions involve more thinking? For computers thought is time...