Test http://localhost/perl/helloworld.pl from your local browser.
If you see the famous 'Hello, world!', congratulations.
If not, check the error log for possible errors:
Sooner or later you will probably use the CGI.pm module.
As indicated here, you need CGI.pm version 3.08 or newer - versions 3.07 and older do not support the new mod_perl API.
Check your CGI.pm version from the command line: