The rewriting behaviour of part identifiers can be configured per handle prefix (actually it can also be done per individual handle but this is not supported for at this point). For EPIC (version 1, so with prefix 11858) the choice was made to rewrite [suffix] to ?[suffix]
So suppose that 11858/1234 resolves to http://clarin.eu then
11858/1234@test=a will be resolved to http://clarin.eu?test=a
Please note that when you offer PIDs with part identifiers that you are responsible of maintaining the part identification fragment as well. Remember that users will use it to link to your resources and that the resulting end point should always be available.