So indeed, it was no illusion that
String::Ruby leaked memory. After I couldn't find anything in my code that would warrant this behaviour it dawned on me that the autobox-patch itself leaked. And it did. It's author forgot to free one SV that was created for each method-call. So it was just a matter of adding
sv_free(autobox_key) in the right spot. Tracking that down however took me a while (with lots of perl recompilations) since I searched in the wrong place.
Chocolate boy (yes, that's what the author calls himself) should have the patch by now.