mod_perl is the more common of the two in our index: it runs on 27 websites against 14 for mod_ssl, and 1 sites use both. Counts come from real scans, not vendor claims.
Mod_perl is an optional module for the Apache HTTP server. It embeds a Perl interpreter into the Apache server. In addition to allowing Apache modules to be written in the Perl programming language, it allows the Apache web server to be dynamically configured by Perl programs.
mod_ssl is an optional module for the Apache HTTP Server. It provides strong cryptography for the Apache web server via the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) cryptographic protocols by the help of the open-source SSL/TLS toolkit OpenSSL.
1 site runs both mod_perl and mod_ssl. That is 7% of the sites using mod_ssl, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.