Slashdot started in 1997, the first release of mod_perl was on 12.03.1998. They certainly used cgi at first, just check the url here: https://web.archive.org/web/19980113191337/http://slashdot.o... - it says cgi right there.

Edit: Seems I was wrong about the date, it was actually 28-Jul-1997[1], nevertheless slashdot was only launched two months later, and as the link above witnesses didn't seem to have used it at first.

[1]: https://web.archive.org/web/19971210053529/http://perl.apach...