const-ness is a hit or miss thing in the perl 5 source. It's generally added if someone thinks about it, but that doesn't happen all that often because we just don't think about it much.

Also, since const-ness tends to propagate downward, you have to be careful adding it in after the fact, otherwise compilers will spew scads of warnings about passing in a const pointer to a routine that expects a normal one. Fixing the warnings generally requires source changes to extensions, and it's a little late to be doing that.