in reply to Regex replace leaking memory.
Note that the copy uses copy-on-write, which allows multiple string SVs to share the same string buffer. In your case, the constant string "TESTING STRING" is shared with $string when the latter is assigned to. When the match is done, the string becomes shared for a second time. When the substitution is then done, $string is modified and its string buffer becomes unshared. This leaves the original buffer shared by the constant SV and the SV in the regex. I think the regex's SV is the one being displayed by Devel::Leak.
Over various releases, perl has differed in how it manages the copy kept by the regex; in particular, when the pattern itself had no captures and $& hadn't yet been seen during compilation perl used to overly-optimise by not capturing, and eval '$&' could return garbage.
Dave.