If client access to/modifying source is an issue, how do you handle it for existing csh scripts? If it's not an issue for csh scripts, why is it an issue for the perl equivelent?

If obfuscating source is a big deal, perl has a compiler or you could obfuscate the source.

RE: Speed
If csh scripts are fast enough for your application, I don't see perl's speed being an issue. If speed is a major issue, you can always consider coding speed critical components in C (as xs) and using perl for the rest.

RE: SQL
Perl has DBI. What is he comparing to DBI? Perl DBD drivers provide useful things like statement caching that provides a speed boost for Oracle and other databases.

RE:HTML and C
What is your ex-boss talking about? Writing CGI stuff in C? mod_perl and apache will give you a platform that can execute nearly as fast as C but with less development time/effort.

RE: Libraries/plug-ins
Has your ex-boss ever visited www.cpan.org? In addition to CGI/mod_perl libraries, you can find modules that will reduce the need to write all your own libraries.

RE: Programmer availability
It's pretty difficult to find good programmers in any language, not just perl, csh, or C.

RE: TK
Perl has an interface to TK. It's been around long enough and is popular enough that OReilly has published books on Perl+TK.

Frankly, I think your ex-boss has a bias against perl (or anything different). Much of his post is FUD against perl.