Well I don't know anymore where I read the word "frame" in connection with foreach-loops.

The glossary of the docs is often far too fuzzy to justify a long discussion...

Anyway grepping through the docs produced

perldiag.pod: (P) The foreach iterator got called in a non-loop context frame.

(I didn't say "call frame")

But the C code is quite hard for me too understand without more conceptual basics... which are fuzzy ...