Now a colleague claims that I cannot rely on this behaviour because constants become *inline* functions and so the lookup might crash in some (unknown?) cases.

Not true.

The inliner does inline most "normal" calls to these subroutines, but since it can't catch every invocation (like soft references, method style invocations, goto &subname, maybe more...), it keeps the sub itself around.

So it means you can use the use constant-generated subs as methods reliably.

After all the inliner is just an optimization, and shouldn't change user-visible behavior.