Ok, this isn't that tricky if you disect the command line switches one at a time:

This means essentially "execute the following code". As you can see, in this particular case, the following code is nothing at all; zilch.

This means wrap the -e code in the following loop:

while( <> ) {
    # -e code goes here (nothing, in this case)
    print $_;
}
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Now for the -i: That means do an in-place edit. Whatever characters immediately follow -i will be appended to the input file's name for the purpose of creating a backup file. Often you'll see -i.bak which means take file foo and create a backup named foo.bak. Well, in your case, instead of .bak you're using ~, so it creates a backup named foo~.

Now put it all together:

1. -i~ Do an in-place edit, but first create a backup file named "inputfile~" (or in your case, foo~).
2. -p loop over the input file (foo), and after executing the -e code, print the line to the output file (which is the same filename as the original input file, because you're doing in-place editing thanks to -i
3. -e Execute the following code within the while loop: '' (in other words, do nothing inside the loop except the -p loop's default; print.

I hope this helps, but if it doesn't, see perlrun for more detail. Also (shameless plug), I wrote a node awhile back that went into greater detail on the subject of learning how to compose Perl one liners. The node is here: Re: One Liners. I think you'll find it to be a pretty easy introduction to Perl's command line switches and Perl one-liners. Good luck!

-i tells Perl that the files read with the diamond operator (<>) are going to be edited in place, and that a backup of the original file will be kept, using the extension specified. So, -i~ will create a copy of the file with a tilde appended to its name.

But, where's the <> coming from? It's from -p, which causes the program to be enclosed into a loop that does a while(<>).

Whether you committed a mistake or not, depends on what you intended to do ;^).

Read perlrun or perldoc perlrun, especially about the -i switch.

Also, you can find the slides for that talk on my homepage.

You might find my article on Perl's command line options to be interesting. And there's always the definitive documentation in perlrun.