I think it spawns a new interpreter right?

Well, eval_sv and eval_pv are related to eval EXPR, not eval BLOCK. There's obviously some performance lost, but not as much as PerlInterpreter * my_perl = perl_alloc(); perl_construct(my_perl); perl_parse(my_perl, NULL, argc, argv, (char **)NULL); perl_run(my_perl); perl_destruct(my_perl); perl_free(my_perl);. New interpreters are constructed for every thread, with tied variables handling access to shared variables, but I don't think a separate interpreter is needed for every eval. eval EXPR additionally spends time in the parser, but eval BLOCK uses the already-parsed data structures.

I've spent a few minutes skimming the source. Perhaps the die() destination and the use feature 'try' entry point could be a good place to start, but I'm not enough of an expert to judge what's really happening here. Though I'm sure it's the same interpreter.