I don't see the claimed problem, it all works as it should

>> Dancer 1.3202 server 2120 listenin
>> Dancer::Plugin::EscapeHTML (0.22)


http://localhost:3000/?jj={%22name%22:%22jsonFileexample%22,%22problem
+%22:%22thisIsThrProblem\%22withComma%22};oj=jjjj
[download]

{
  jj => "{\"name\":\"jsonFileexample\",\"problem\":\"thisIsThrProblem\\\"withComma\"}",
  oj => "jjjj",
}

use Dancer;
use Data::Dump qw/ pp /;
use Dancer::Plugin::EscapeHTML;
any '/' => sub {
    "<pre>".escape_html( pp( scalar params() ) )."</pre>";;;;
};
dance;
[download]