You want Apache::DB.

Put the following in your httpd.conf:

# conditionally load the perl debugger
<IfDefine PERLDB>
  <Perl>
    use Apache::DB ();
    Apache::DB->init;
  </Perl>
  
  <Location />
    PerlInitHandler Apache::DB
  </Location>
</IfDefine>
[download]

Then start apache like so:

Instead of returning to the terminal like it usually does, the perl debugger will start. Then you can browse around your site. Now you can set breakpoints, step through code, inspect variables, or whatever.

The only way I've found to stop the debugger is with kill -9 PID, so you'll need another terminal.

If you are using mp2, there are a couple additional settings you need. See the link above or run perldoc Apache::DB for the details.

Hope this helps,

trwww