This site tracks bugs in every distribution released through CPAN. All bug reports are publicly visible on this site. If a distribution tracks it's bugs elsewhere, that will also be noted on the pages for that distribution here.
... find a distribution and browse its bugs?
This site lets you search for distributions by name or author. When you've found the distribution you're looking for, you can browse known bugs and report new bugs.
... report a bug in a module using web-interface?
Use one of the authentication options listed here to log in. Auth0 provides several options including Google and Github logins. Module authors can use PAUSE credentials to manage their queues.
... report a bug in a module using email?
To submit a bug report for a given distribution by email, send mail to bug-<distribution-name>@rt.cpan.org, where "<distribution-name>" is something like DBIx-SearchBuilder or Class-DBI or Acme-Current-Forever. Use search to find name of a distribution.
Note: The contents of any email you send will be publicly visible on the web.
... reply to an existing bug via email?
Send email to the distribution email address (see above) with a tag like the following somewhere in the Subject: [rt.cpan.org #NNNNN]. Replace NNNNN with the existing bug number.
... report a bug in perl itself?
To report bugs in perl itself, you're best off using the command-line 'perlbug' tool or going to the GitHub issue tracker.
... manage bugs in your distributions?
To work with bugs, every module author with a PAUSE account can log into rt.cpan.org with their PAUSE userid and password. If you can't log in or distributions you maintain aren't listed, you can send email to firstname.lastname@example.org.