Actually, you're shielded from the URL only if you use certain methods. show_user is not one of them: you need to specify a user, and a format in the URL.
I should be setting apiurl instead of apihost. Confusion resulting from using apihost in PHP with curl. In any case, I figured out why I kept getting error 500. I took out all code, except for a statement to print out Hello World. When I commented out the use Net::Twitter, it printed out fine, and when I put it back in, I got error 500. This happened on a hosting account I have on goDaddy and another I have on a generic Apache Host. So, it seems as if Net::Twitter is not in general use as a Perl module.