Unless you start messing around with print (which I wouldn't recommend until you understand CGI::App more), CGI::App will only display what you explictly return from a runmode.

So to show the results of another sub that your runmode calls, you need to get it's results into your runmode's return:-

# Perhaps like this...

return errorhandler(\@errors) 
     . "here2: " . Dumper(@errors);

# ...or this...

my $errors = errorhandler(\@errors);
return $errors . "here2: " . Dumper(@errors);
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Update: s/would/wouldn't/;