I've never seen someone try to use set_die_handler, so if you're trying to do it all yourself, forget all about CGI::Carp and use do

BEGIN { $main::SIG{__DIE__}  = \&extra_special_die_handler; }
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The double/output is bugs in CGI::Carp

its either inadequate documentation explaining how to best use this feature

logic error in "sub CGI::Carp::die" in that it realdie's after calling die_handler even though that will just call diehandler again

no signal/sentinel to die_handler that its being called by realdie

or still overriding CORE::die with CGI::Carp::die despite using set_die_handler, so having die_handler get called twice

could be argued (i feel) set_die_handler should not exist at all -- $main::SIG{__DIE__} is known/documented in perlvar