I can confirm that you have to run

mine = Perl_get_context();
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before you can use this. The run of

dSP;
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just segfaults if

mine
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have not been set via

mine = Perl_get_context();
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I call on initialisation of my module a XS function which does does do

mine = Perl_get_context();
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and verify that mine is not NULL before trying to call a perl function.

All I needed is the following to get it working, and call the function "functionname" within of the module I called "setcallback" from.
The use in C:

void dorunperlfunction() {
   if (mine) {
      PERL_SET_CONTEXT(mine);
      dSP;
      ENTER;
      SAVETMPS;
      PUSHMARK(SP);
      PUTBACK;
      call_pv("functionname", G_DISCARD|G_NOARGS);
      FREETMPS;
      LEAVE;
   }
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The initialisation in XS(!):

PerlInterpreter * mine = NULL;
...
void
setcallback()
   CODE:
      ...
      mine = Perl_get_context();
      ...
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