The diagnostics pragma is a part of the Perl distribution. See perldoc diagnostics for more information about the program, and perldoc perldiag for Perl diagnostics; this is the source of this pragma's information.
ab(1) (ApacheBench) comes bundled with the Apache web server and is available from http://httpd.apache.org/.
httperf(1) is available from http://www.hpl.hp.com/personal/David_Mosberger/httperf.html.
http_load(1) is available from http://www.acme.com/software/http_load/.
BenchWeb (http://www.netlib.org/benchweb/) is a good starting point for finding information about computer system performance benchmarks, benchmark results, and benchmark code.
The libgtop library (ftp://ftp.gnome.org/pub/GNOME/sources/gtop/) is a part of the GNOME project (http://www.gnome.org/). Also try http://fr.rpmfind.net/linux/rpm2html/search.php?query=libgtop.
Chapter 3 of Web Performance Tuning, by Patrick Killelea (O'Reilly).
Chapter 9 of mod_perl Developer's Cookbook, by Geoffrey Young, Paul Lindner, and Randy Kobes (Sams).