If you decide to go with a utility solution rather than a Perl solution, an alternative to using `diff` is `comm`, which finds common lines in sorted files. Each line is put into one of three columns depending on whether it is in the first file, the second file, or both files. (There is no column for lines that are in neither file.) The command line arguments let you turn off columns you don't want.

comm -23 newsite1.txt newsite2.txt will print lines that are only in newsite1.txt, and
comm -13 newsite1.txt newsite2.txt will print lines that are only in newsite2.txt

comm -3 newsite1.txt newsite2.txt will print lines that in are only in newsite1.txt in the first column, and lines that are only in newsite2.txt in the second column.