I learnt this the hard way. I used to write:

if ( $DEBUG ){
  use CGI::Carp qw(fatalsToBrowser);
}
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thinking that when I put the code into production all my debugging would dissapear with a simple $DEBUG=0; but Carp is always loaded no matter the value of $DEBUG. It took a long while for me to notice the error becuase when run in prod the conditions which would trigger a die didn't normally occur. My bad.