I've had trouble accessing .pl files on some of my sites when using Internet Explorer from a machine that had perl installed on it. The browser would occassionally try to execute whatever was returned by the CGI program because it thinks .pl means a perl program so it tries to run it. When I run into this, I rename them to .cgi. This allows me to figure out what the real problem is without the browser getting in the way (Windows doesn't know what a .cgi file is). All I have to do is put a little directive in my .htaccess file to tell the web server that .cgi files should be processed with perl.

If you wanted to, you could modify your own web server's configuration files to change the file extension mappings. You could write your web site in perl using files with .php extensions so long as the web server thinks .php files should be processed with perl. You could also write your web site in php with .pl extensions so long as you tell your web server to process .pl files with the php interpreter. It really doesn't matter what the extension is so long as the web server knows how to map it to the right processor.

I'll agree with the first reply in that none of this should matter at all to search engines unless they are trying to guess what language your site is written in.

OT Rant of course. IE chokes on .pl files when Perl is installed on a target machine!? That's insane! I cannot believe that Microsoft still refuses to make IE respect the Content-Type header. Why do people still bother to use this piece of trash?? /rant
Lobster Aliens Are attacking the world!

FWIW, I don't think it makes a bit of difference to search engines.

The choice of language is irrelevant.

as others have said, it doesn't matter what your pages are written in. all clients should care about is the Content-Type of the results.

that said, it is highly recommended that you make use of technology like mod_rewrite to eliminate filename extensions from urls. this will help you when, a few years down the line, you decide to rewrite your site in a different language. if you have to change urls, you'll break other sites' links (and lose PageRank with google).

search engines are also usually a little skittish about indexing content that looks dynamically generated. this is based not on filename extensions but by parameters in the url (ie if there is a '?' with anything after it). eg, google will follow links to a page with parameters, but won't follow links from that page to other dynamic pages. this is to avoid getting the spider caught in an infinite loop (imagine if a site appended a random number to every url.) again, using something like mod_rewrite to change your urls from 'product.pl?product_id=1234' to something more like '/products/1234/' will make it much easier to index.