CPAN: All of the Perl-related software mentioned in this article is available via CPAN, the Comprehensive Perl Archive Network. You can access it at http://www.perl.com/CPAN/. That should automatically forward you to the CPAN mirror nearest your house, although it may need a bit of help in doing so. From CPAN, you can download mod_perl (the Perl module for Apache), HTML-Embperl (the package in which Embperl comes) and many other goodies relating to Perl and the Web.
Particularly useful are the modules that mod_perl and Embperl require in order to function: LWP, HTML::HeadParser, CGI.pm, and MIME::Base64. If you find yourself installing CPAN software frequently, you might want to consider the CPAN module (CPAN.pm), which provides a nice interface for installing and updating software distributed via CPAN.
ePerl is another HTML/Perl hybrid template system that I haven't looked at very closely. You can download it from http://www.engelschall.com/sw/eperl/.
Text::Template was one of the first Perl modules for creating Perl/HTML templates, and still does the trick for the simple stuff. Don't be scared by its relatively low version number; while it doesn't do everything, it should be more than adequate for most needs. Text::Template seems to encourage writing large blocks of Perl code, in contrast with Embperl's one-line style. You can get Text::Template from CPAN.
PHP works similarly to Embperl, except that it uses its own language (rather than Perl) inside of the code blocks. A PHP module is available for mod_perl, and it comes with support for a large number of database servers, pseudo-persistent connections and similar features. More information, as well as the software, is available from http://www.php.net/.