[perl #39864] [BUG] Compiling P6Regex within code substitutes part of string to match away

[Leopold Toetsch via RT]

as mentioned on IRC, it looks like a GC bug, but
actually it was a COW string bug (a possibly already set live flag wasn't
 cleared, which lead to half-moved string memory, s. resources.c:370)

Fixed w. r13400