my $cmdout = join '', <DATA>; is the same as if I just put all the stuff below the __DATA__ line into $cmdout directly. It's also the same as if I had that data in a file, opened a filehandle to it (let's say FH) and then read it using my $cmdout = join '', <FH>;. Basically, __DATA__ is just a good way to post code examples without having to exclude the file data it's running on.