CGI::Carp versions before 5.00 for Perl leak warnings across requests in persistent environments via the warningsToBrowser buffer.
Warnings are held in a package-level buffer until warningsToBrowser writes them into the page as HTML comments, and the buffer is never cleared between requests. Under mod_perl, FastCGI or PSGI, warnings from one request are written into a later one.
Warnings that contain one user's request data can appear in the HTML sent to another user.
For deployments that cannot upgrade, do not use warningsToBrowser in persistent environments. After upgrading, persistent applications where nothing calls CGI::initialize_globals between requests must also call CGI::Carp::_reset_globals at the start of each request.
Upgrade to CGI 5.00 or later.